CHAPTER XIII

THE EASTERN ANDES: THE CORDILLERA VILCAPAMPA

The culminating range of the eastern Andes is the so-called Cordillera Vilcapampa. Its numerous, sharp, snow-covered peaks are visible in every summit view from the central portion of the Andean system almost to the western border of the Amazon basin. Though the range forms a water parting nearly five hundred miles long, it is crossed in several places by large streams that flow through deep canyons bordered by precipitous cliffs. The Urubamba between Torontoy and Colpani is the finest illustration. For height and ruggedness the Vilcapampa mountains are among the most noteworthy in Peru. Furthermore, they display glacial features on a scale unequaled elsewhere in South America north of the ice fields of Patagonia.

GLACIERS AND GLACIAL FORMS

One of the most impressive sights in South America is a tropical forest growing upon a glacial moraine. In many places in eastern Bolivia and Peru the glaciers of the Ice Age were from 5 to 10 miles long--almost the size of the Mer de Glace or the famous Rhone glacier. In the Juntas Valley in eastern Bolivia the tree line is at 10,000 feet (3,050 m.), but the terminal moraines lie several thousand feet lower. In eastern Peru the glaciers in many places extended down nearly to the tree line and in a few places well below it. In the Cordillera Vilcapampa vast snowfields and glacier systems were spread out over a summit area as broad as the Southern Appalachians. The snowfields have since shrunk to the higher mountain recesses; the glaciers have retreated for the most part to the valley heads or the cirque floors; and the lower limit of perpetual snow has been raised to 15,500 feet.

These features are surprising because neither Whymper[44] nor Wolf[45] mentions the former greater extent of the ice on the volcanoes of Ecuador, only ten or twelve degrees farther north. Moreover, Reiss[46] denies that the hypothesis of universal climatic change is supported by the facts of a limited glaciation in the High Andes of Ecuador; and J. W. Gregory[47] completely overlooks published proof of the existence of former more extensive glaciers elsewhere in the Andes:

“... the absence not only of any traces of former more extensive glaciation from the tropics, as in the Andes and Kilimandjaro, but also from the Cape.” He says further: “In spite of the extensive glaciers now in existence on the higher peaks of the Andes, there is practically no evidence of their former greater extension.”(!)

Whymper spent most of his time in exploring recent volcanoes or those recently in eruption, hence did not have the most favorable opportunities for gathering significant data. Reiss was carried off his feet by the attractiveness of the hypothesis[48] relating to the effect of glacial denudation on the elevation of the snowline. Gregory appeared not to have recognized the work of Hettner on the Cordillera of Bogotá and of Sievers[49] and Acosta on the Sierra Nevada de Santa Marta in northern Colombia.

The importance of the glacial features of the Cordillera Vilcapampa developed on a great scale in very low latitudes in the southern hemisphere is twofold: first, it bears on the still unsettled problem of the universality of a colder climate in the Pleistocene, and, second, it supplies additional data on the relative depression of the snowline in glacial times in the tropics. Snow-clad mountains near the equator are really quite rare. Mount Kenia rising from a great jungle on the equator, Kilimandjaro with its two peaks, Kibo and Mawenzi, two hundred miles farther south, and Ingomwimbi in the Ruwenzori group thirty miles north of the equator, are the chief African examples. A few mountains from the East Indies, such as Kinibalu in Borneo, latitude 6° north, have been found glaciated, though now without a snow cover. In higher latitudes evidences of an earlier extensive glaciation have been gathered chiefly from South America, whose extension 13° north and 56° south of the equator, combined with the great height of its dominating Cordillera, give it unrivaled distinction in the study of mountain glaciation in the tropics.

Furthermore, mountain summits in tropical lands are delicate climatic registers. In this respect they compare favorably with the inclosed basins of arid regions, where changes in climate are clearly recorded in shoreline phenomena of a familiar kind. Lofty mountains in the tropics are in a sense inverted basins, the lower snowline of the past is like the higher shoreline of an interior basin; the terminal moraines and the alluvial fans in front of them are like the alluvial fans above the highest strandline; the present snow cover is restricted to mountain summits of small areal extent, just as the present water bodies are restricted to the lowest portions of the interior basin; and successive retreatal stages are marked by terminal moraines in the one case as they are marked in the other by flights of terraces and beach ridges.

I made only a rapid reconnaissance across the Cordillera Vilcapampa in the winter season, and cannot pretend from my limited observations to solve many of the problems of the field. The data are incorporated chiefly in the chapter on Glacial Features. In this place it is proposed to describe only the more prominent glacial features, leaving to later expeditions the detailed descriptions upon which the solution of some of the larger problems must depend.

At Choquetira three prominent stages in the retreat of the ice are recorded. The lowermost stage is represented by the great fill of morainic and outwash material at the junction of the Choquetira, and an unnamed valley farther south at an elevation of 11,500 feet (3,500 m.). A mile below Choquetira a second moraine appears, elevation 12,000 feet (3,658 m.), and immediately above the village a third at 12,800 (3,900 m.). The lowermost moraine is well dissected, the second is ravined and broken but topographically distinct, the third is sharp-crested and regular. A fourth though minor stage is represented by the moraine at the snout of the living glacier and still less important phases are represented in some valleys--possibly the record of post-glacial changes of climate. Each main moraine is marked by an important amount of outwash, the first and third moraines being associated with the greatest masses. The material in the moraines represents only a part of that removed to form the successive steps in the valley profile. The lowermost one has an enormous volume, since it is the oldest and was built at a time when the valley was full of waste. It is fronted by a deep fill, over the dissected edge of which one may descend 800 feet in half an hour. It is chiefly alluvial in character, whereas the next higher one is composed chiefly of bowlders and is fronted by a pronounced bowlder train, which includes a remarkable perched bowlder of huge size. Once the valley became cleaned out the ice would derive its material chiefly by the slower process of plucking and abrasion, hence would build much smaller moraines during later recessional stages, even though the stages were of equivalent length.

There is a marked difference in the degree of dissection of the moraines. The lowermost and oldest is so thoroughly dissected as to exhibit but little of its original surface. The second has been greatly modified, but still possesses a ridge-like quality and marks the beginning of a noteworthy flattening of the valley gradient. The third is as sharp-crested as a roof, and yet was built so long ago that the flat valley floor behind it has been modified by the meandering stream. From this point the glacier retreated up-valley several miles (estimated) without leaving more than the thinnest veneer on the valley floor. The retreat must, therefore, have been rapid and without even temporary halts until the glacier reached a position near that occupied today. Both the present ice tongues and snowfields and those of a past age are emphasized by the presence of a patch of scrub and woodland that extends on the north side of the valley from near the snowline down over the glacial forms to the lower valley levels.

The retreatal stages sketched above would call for no special comment if they were encountered in mountains in northern latitudes. They would be recognized at once as evidence of successive periodic retreats of the ice, due to successive changes in temperature. To understand their importance when encountered in very low latitudes it is necessary to turn aside for a moment and consider two rival hypotheses of glacial retreat. First we have the hypothesis of periodic retreat, so generally applied to terminal moraines and associated outwash in glaciated mountain valleys. This implies also an advance of the ice from a higher position, the whole taking place as a result of a climatic change from warmer to colder and back again to warmer.

But evidences of more extensive mountain glaciation in the past do not in themselves prove a change in climate over the whole earth. In an epoch of fixed climate a glacier system may so deeply and thoroughly erode a mountain mass, that the former glaciers may either diminish in size or disappear altogether. As the work of excavation proceeds, the catchment basins are sunk to, and at last below, the snowline; broad tributary spurs whose snows nourish the glaciers, may be reduced to narrow or skeleton ridges with little snow to contribute to the valleys on either hand; the glaciers retreat and at last disappear. There would be evidences of glaciation all about the ruins of the former loftier mountain, but there would be no living glaciers. And yet the climate might remain the same throughout.

It is this “topographic” hypothesis that Reiss and Stübel accept for the Ecuadorean volcanoes. Moreover, the volcanoes of Ecuador are practically on the equator--a very critical situation when we wish to use the facts they exhibit in the solution of such large problems as the contemporaneous glaciation of the two hemispheres, or the periodic advance and retreat of the ice over the whole earth. This is not the place to scrutinize either their facts or their hypothesis, but I am under obligations to state very emphatically that the glacial features of the Cordillera Vilcapampa require the climatic and not the topographic hypothesis. Let us see why.

The differences in degree of dissection and the flattening gradient up-valley that we noted in a preceding paragraph leave no doubt that each moraine of the bordering valleys in the Vilcapampa region, represents a prolonged period of stability in the conditions of topography as well as of temperature and precipitation. If change in topographic conditions is invoked to explain retreat from one position to the other there is left no explanation of the periodicity of retreat which has just been established. If a period of cold is inaugurated and glaciers advance to an ultimate position, they can retreat only through change of climate effected either by general causes or by topographic development to the point where the snowfields become restricted in size. In the case of climatic change the ice changes are periodic. In the case of retreat due to topographic change there should be a steady or non-periodic falling back of the ice front as the catchment basins decrease in elevation and the snow-gathering ridges tributary to them are reduced in height.

Further, the matterhorns of the Cordillera Vilcapampa are not bare but snow-covered, vigorous glaciers several miles in length and large snowfields still survive and the divides are not arêtes but broad ridges. In addition, the last two moraines, composed of very loose material, are well preserved. They indicate clearly that the time since their formation has witnessed no wholesale topographic change. If (1) no important topographic changes have taken place, and (2) a vigorous glacier lay for a long period back of a given moraine, and (3) _suddenly retreated several miles and again became stable_, we are left without confidence in the application of the topographic hypothesis to the glacial features of the Vilcapampa region. Glacial retreat may be suddenly begun in the case of a late stage of topographic development, but it should be an orderly retreat marked by a large number of small moraines, or at least a plentiful strewing of the valley floor with débris.

The number of moraines in the various glaciated valleys of the Cordillera Vilcapampa differ, owing to differences in elevation and to the variable size of the catchment basins. All valleys, however, display the same sudden change from moraine to moraine and the same characteristics of gradient. In all of them the lowermost moraine is always more deeply eroded than the higher moraines, in all of them glacial erosion was sufficiently prolonged greatly to modify the valley walls, scour out lake basins, or broad flat valley floors, develop cirques, arêtes, and pinnacled ridges in limited number. In some, glaciation was carried to the point where only skeleton divides remained, in most places broad massive ridges or mountain knots persist. In spite of all these differences successive moraines were formed, separated by long stretches either thinly covered with till or exposing bare rock.

In examining this group of features it is important to recognize the essential fact that though the number of moraines varies from valley to valley, the differences in character between the moraines at low and at high elevations in a single valley are constant. It is also clear that everywhere the ice retreated and advanced periodically, no matter with what topographic features it was associated, whether those of maturity or of youth in the glacial cycle. We, therefore, conclude that topographic changes had no significant part to play in the glacial variations in the Cordillera Vilcapampa.

The country west of the Cordillera Vilcapampa had been reduced to early topographic maturity before the Ice Age, and then uplifted with only moderate erosion of the masses of the interfluves. That on the east had passed through the same sequence of events, but erosion had been carried much farther. The reason for this is found in a strong climatic contrast. The eastern is the windward aspect and receives much more rain than the western. Therefore, it has more streams and more rapid dissection. The result was that the eastern slopes were cut to pieces rapidly after the last great regional uplift; the broad interfluves were narrowed to ridges. The region eastward from the crest of the Cordillera to the Pongo de Mainique looks very much like the western half of the Cascade Mountains in Oregon--the summit tracts of moderate declivity are almost all consumed.

The effect of these climatic and topographic contrasts is manifested in strong contrasts in the position and character of the glacial forms on the opposite slopes of the range. At Pampaconas on the east the lowermost terminal moraine is at least a thousand feet below timber line. Between Vilcabamba pueblo and Puquiura the terminal moraine lies at 11,200 feet (3,414 m.). By contrast the largest Pleistocene glacier on the western slope, nearly twelve miles long, and the largest along the traverse, ended several miles below Choquetira at 11,500 feet (3,504 m.) elevation, or just at the timber line. Thus, the steeper descents of the eastern side of the range appear to have carried short glaciers to levels far lower than those attained by the glaciers of the western slope.

It seems at first strange that the largest glaciers were west of the divide between the Urubamba and the Apurimac, that is, on the relatively dry side of the range. The reason lies in a striking combination of topographic and climatic conditions. Snow is a mobile form of precipitation that is shifted about by the wind like a sand dune in the desert. It is not required, like water, to begin a downhill movement as soon as it strikes the earth. Thus, it is a noteworthy fact that snow drifting across the divides may ultimately cause the largest snowfields to lie where the least snow actually falls. This is illustrated in the Bighorns of Wyoming and others of our western ranges. It is, however, not the wet snow near the snowline, but chiefly the dry snow of higher altitudes that is affected. What is now the dry or leeward side of the Cordillera appears in glacial times to have actually received more snow than the wet windward side.

The topography conspired to increase this contrast. In place of many streams, direct descents, a dispersion of snow in many valleys, as on the east, the western slopes had indirect descents, gentler valley profiles, and that higher degree of concentration of drainage which naturally goes with topographic maturity. For example, there is nothing in the east to compare with the big spurless valley near the pass above Arma. The side walls were so extensively trimmed that the valley was turned into a trough. The floor was smoothed and deepened and all the tributary glaciers were either left high up on the bordering slopes or entered the main valley with very steep profiles; their lateral and terminal moraines now hang in festoons on the steep side walls. Moreover, the range crest is trimmed from the west so that the serrate skyline is a feature rarely seen from eastern viewpoints. This may not hold true for more than a small part of the Cordillera. It was probably emphasized here less by the contrasts already noted than by the geologic structure. The eastward-flowing glaciers descended over dip slopes on highly inclined sandstones, as at Pampaconas. Those flowing westward worked either in a jointed granite or on the outcropping _edges_ of the sandstones, where the quarrying process known as glacial plucking permitted the development of excessively steep slopes.

There are few glacial steps in the eastern valleys. The western valleys have a marvelous display of this striking glacial feature. The accompanying hachure maps show them so well that little description is needed. They are from 50 to 200 feet high. Each one has a lake at its foot into which the divided stream trickles over charming waterfalls. All of them are clearly associated with a change in the volume of the glacier that carved the valley. Wherever a tributary glacier entered, or the side slopes increased notably in area, a step was formed. By retreat some of them became divided, for the process once begun would push the step far up valley after the manner of an extinguishing waterfall.

The retreat of the steps, the abrasion of the rock, and the sapping of the cirques at the valley heads excavated the upper valleys so deeply that they are nearly all, as W. D. Johnson has put it, “down at the heel.” Thus, above Arma, one plunges suddenly from the smooth, grassy glades of the strongly glaciated valley head down over the outer slopes of the lowermost terminal moraine to the steep lower valley. Above the moraine are fine pastures, in the steep valley below are thickets and rocky defiles. There are long quiet reaches in the streams of the glaciated valley heads besides pretty lakes and marshes. Below, the stream is swift, almost torrential. Arma itself is built upon alluvial deposits of glacial origin. A mile farther down the valley is constricted and steep-walled--really a canyon.

Though the glaciers have retreated to the summit region, they are by no means nearing extinction. The clear blue ice of the glacier descending from Mt. Soiroccocha in the Arma Valley seems almost to hang over the precipitous valley border. In curious contrast to its suggestion of cold and storm is the patch of dark green woodland which extends right up to its border. An earthquake might easily cause the glacier to invade the woodland. Some of the glaciers between Choquetira and Arma rest on terminal moraines whose distal faces are from 200 to 300 feet high. The ice descending southeasterly from Panta Mt. is a good illustration. Earlier positions of the ice front are marked by equally large moraines. The one nearest that engaged by the living glacier confines a large lake that discharges through a gap in the moraine and over a waterfall to the marshy floor of the valley.

Retreat has gone so far, however, that there are only a few large glacier systems. Most of the tributaries have withdrawn toward their snowfields. In place of the twenty distinct glaciers now lying between the pass and the terminal moraine below Choquetira, there was in glacial times one great glacier with twenty minor tributaries. The cirques now partly filled with damp snow must then have been overflowing with dry snow above and ice below. Some of the glaciers were over a thousand feet thick; a few were nearly two thousand feet thick, and the cirques that fed them held snow and ice at least a half mile deep. Such a remarkably complete set of glacial features only 700 miles from the equator is striking evidence of the moist climate on the windward eastern part of the great Andean Cordillera, of the universal change in climate in the glacial period, and of the powerful dominating effects of ice erosion in this region of unsurpassed Alpine relief.

THE VILCAPAMPA BATHOLITH AND ITS TOPOGRAPHIC EFFECTS

The main axis of the Cordillera Vilcapampa consists of granite in the form of a batholith between crystalline schists on the one hand (southwest), and Carboniferous limestones and sandstones and Silurian shales and slates on the other (northeast). It is not a domal uplift in the region in which it was observed in 1911, but an axial intrusion, in places restricted to a narrow belt not more than a score of miles across. As we should expect from the variable nature of the invaded material, the granite belt is not uniform in width nor in the character of its marginal features. In places the intrusion has produced strikingly little alteration of the country rock; in other localities the granite has been injected into the original material in so intimate a manner as almost completely to alter it, and to give rise to a very broad zone of highly metamorphosed rock. Furthermore, branches were developed so that here and there tributary belts of granite extend from the main mass to a distance of many miles. Outlying batholiths occur whose common petrographic character and similar manner of occurrence leave little doubt that they are related abyssally to a common plutonic mass.

The Vilcapampa batholith has two highly contrasted borders, whether we consider the degree of metamorphism of the country rock, the definition of the border, or the resulting topographic forms. On the northeastern ridge at Colpani the contact is so sharp that the outstretched arms in some places embrace typical granite on the one hand and almost unaltered shales and slates on the other. Inclusions or xenoliths of shale are common, however, ten and fifteen miles distant, though they are prominent features in a belt only a few miles wide. The lack of more intense contact effects is a little remarkable in view of the altered character of the inclusions, all of which are crystalline in contrast to the fissile shales from which they are chiefly derived. Inclusions within a few inches of the border fall into a separate class, since they show in general but trifling alteration and preserve their original cleavage plains. It appears that the depth of the intrusion must have been relatively slight or the intrusion sudden, or both shallow and sudden, conditions which produce a narrow zone of metamorphosed material and a sharp contact.

The relation between shale and granite at Colpani is shown in Fig. 143. Projections of granite extend several feet into the shale and slate and generally end in blunt barbs or knobs. In a few places there is an intimate mixture of irregular slivers and blocks of crystallized sediments in a granitic groundmass, with sharp lines of demarcation between igneous and included material. The contact is vertical for at least several miles. It is probable that other localities on the contact exhibit much greater modification and invasion of the weak shales and slates, but at Colpani the phenomena are both simple and restricted in development.

The highly mineralized character of the bordering sedimentary strata, and the presence of numbers of complementary dikes, nearly identical in character to those in the parent granite now exposed by erosion over a broad belt roughly parallel to the contact, supplies a basis for the inference that the granite may underlie the former at a slight depth, or may have had far greater metamorphic effects upon its sedimentary roof than the intruded granite has had upon its sedimentary rim.

The physiographic features of the contact belt are of special interest. No available physiographic interpretation of the topography of a batholith includes a discussion of those topographic and drainage features that are related to the lithologic character of the intruded rock, the manner of its intrusion, or the depth of erosion since intrusion. Yet each one of these factors has a distinct topographic effect. We shall, therefore, turn aside for a moment from the detailed discussion of the Vilcapampa region to an examination of several physiographic principles and then return to the main theme for applications.

It is recognized that igneous intrusions are of many varieties and that even batholithic invasions may take place in rather widely different ways. Highly heated magmas deeply buried beneath the earth’s surface produce maximum contact effects, those nearer the surface may force the strata apart without extreme lithologic alterations of the displaced beds, while through the stoping process a sedimentary cover may be largely absorbed and the magmas may even break forth at the surface as in ordinary vulcanism. If the sedimentary beds have great vertical variation in resistance, in attitude, and in composition, there may be afforded an opportunity for the display of quite different effects at different levels along a given contact, so that a great variety of physical conditions will be passed by the descending levels of erosion. At one place erosion may have exposed only the summit of the batholith, at another the associated dikes and sheets and ramifying branches may be exposed as in the zone of fracture, at a third point the original zone of flowage may be reached with characteristic marginal schistosity, while at still greater depths there may be uncovered a highly metamorphosed rim of resistant sedimentary rock.

The mere enumeration of these variable structural features is sufficient to show how variable we should expect the associated land forms to be. Were the forms of small extent, or had they but slight distinction upon comparison with other erosional effects, they would be of little concern. They are, on the contrary, very extensively developed; they affect large numbers of lofty mountain ranges besides still larger areas of old land masses subjected to extensive and deep erosion, thus laying bare many batholiths long concealed by a thick sedimentary roof.

The differences between intruded and country rock dependent upon these diversified conditions of occurrence are increased or diminished according to the history of the region after batholithic invasion takes place. Regional metamorphism may subsequently induce new structures or minimize the effects of the old. Joint systems may be developed, the planes widely spaced in one group of rocks giving rise to monolithic masses very resistant to the agents of weathering, while those of an adjacent group may be so closely spaced as greatly to hasten the rate of denudation. There may be developed so great a degree of schistosity in one rock as to give rise (with vigorous erosion) to a serrate topography; on the other hand the forms developed on the rocks of a batholith may be massive and coarse-textured.

To these diversifying conditions may be added many others involving a large part of the field of dynamic geology. It will perhaps suffice to mention two others: the stage of erosion and the special features related to climate. If a given intrusion has been accompanied by an important amount of uplift or marginal compression, vigorous erosion may follow, whereupon a chance will be offered for the development of the greatest contrast in the degree of boldness of topographic forms developed upon rocks of unequal resistance. Ultimately these contrasts will diminish in intensity, as in the case of all regional differences of relief, with progress toward the end of the normal cycle of erosion. If peneplanation ensue, only feeble topographic differences may mark the line of contact which was once a prominent topographic feature. With reference to the effects of climate it may be said simply that a granite core of batholithic origin may extend above the snowline or above timber line or into the timbered belt, whereas the invaded rock may occur largely below these levels with obvious differences in both the rate and the kind of erosion affecting the intruded mass.

If we apply the foregoing considerations to the Cordillera Vilcapampa, we shall find some striking illustrations of the principles involved. The invasion of the granite was accompanied by moderate absorption of the displaced rock, and more especially by the marginal pushing aside of the sedimentary rim. The immediate effect must have been to give both intruded rock and country rock greater height and marked ruggedness. There followed a period of regional compression and torsion, and the development of widespread joint systems with strikingly regular features. In the Silurian shales and slates these joints are closely spaced; in the granites they are in many places twenty to thirty feet apart. The shales, therefore, offer many more points of attack and have weathered down into a smooth-contoured topography boldly overlooked along the contact by walls and peaks of granite. _In some cases a canyon wall a mile high is developed entirely on two or three joint planes inclined at an angle no greater than 15°._ The effect in the granite is to give a marked boldness of relief, nowhere more strikingly exhibited than at Huadquiña, below Colpani, where the foot-hill slopes developed on shales and slates suddenly become moderate. The river flows from a steep and all but uninhabited canyon into a broad valley whose slopes are dotted with the terraced _chacras_, or farms, of the mountain Indians.

The Torontoy granite is also homogeneous while the shales and slates together with their more arenaceous associates occur in alternating belts, a diversity which increases the points of attack and the complexity of the forms. Tending toward the same result is the greater hardness of the granite. The tendency of the granite to develop bold forms is accelerated in lofty valleys disposed about snow-clad peaks, where glaciers of great size once existed, and where small glaciers still linger. The plucking action of ice has an excellent chance for expression, since the granite may be quarried cleanly without the production of a large amount of spoil which would load the ice and diminish the intensity of its plucking action.

As a whole the Central Andes passed through a cycle of erosion in late Tertiary time which was interrupted by uplift after the general surface had been reduced to a condition of topographic maturity. Upon the granites mature slopes are not developed except under special conditions (1) of elevation as in the small batholith above Chuquibambilla, and (2) where the granite is itself bordered by resistant schists which have upheld the surface over a broad transitional belt. Elsewhere the granite is marked by exceedingly rugged forms: deep steep-walled canyons, precipitous cirques, matterhorns, and bold and extended escarpments of erosion. In the shale belt the trails run from valley to valley in every direction without special difficulties, but in the granite they follow the rivers closely or cross the axis of the range by carefully selected routes which generally reach the limit of perpetual snow. Added interest attaches to these bold topographic forms because of the ruins now found along the canyon walls, as at Torontoy, or high up on the summit of a precipitous spur, as at Machu Picchu near the bridge of San Miguel.

The Vilcapampa batholith is bordered on the southwest by a series of ancient schists with which the granite sustains quite different relations. No sharp dividing line is visible, the granite extending along the planes of foliation for such long distances as in places to appear almost interbedded with the schists. The relation is all the more striking in view of the trifling intrusions effected in the case of the seemingly much weaker shales on the opposite contact. Nor is the metamorphism of the invaded rock limited to simple intrusion. For several miles beyond the zone of intenser effects the schists have been enriched with quartz to such an extent that their original darker color has been changed to light gray or dull white. At a distance they may even appear as homogeneous and light-colored as the granite. At distant points the schists assume a darker hue and take on the characters of a rather typical mica schist.

It is probable that the Vilcapampa intrusion is one of a family of batholiths which further study may show to extend over a much larger territory. The trail west of Abancay was followed quite closely and accidentally crosses two small batholiths of peculiar interest. Their limits were not closely followed out, but were accurately determined at a number of points and the remaining portion of the contact inferred from the topography. In the case of the larger area there may indeed be a connection westward with a larger mass which probably constitutes the ranges distant some five to ten miles from the line of traverse.

These smaller intrusions are remarkable in that they appear to have been attended by little alteration of either invading or invaded rock, though the granites were observed to become distinctly more acid in the contact zone. Space was made for them by displacing the sedimentary cover and by a marked shortening of the sedimentary rim through such structures as overthrust faults and folds. The contact is observable in a highly metamorphosed belt about twenty feet wide, and for several hundred feet more the granite has absorbed the limestone in small amounts with the production of new minerals and the development of a distinctly lighter color. The deformative effects of the batholithic invasion are shown in their gross details in Figs. 141, 142, and 146; the finer details of structure are represented in Fig. 147, which is drawn from a measured outcrop above Chuquibambilla.

It will be seen that we have here more than a mere crinkling, such as the mica schists of the Cordillera Vilcapampa display. The diversified sedimentary series is folded and faulted on a large scale with broad structural undulations visible for miles along the abrupt valley walls. Here and there, however, the strata become weaker generally through the thinning of the beds and the more rapid alternation of hard and soft layers, and for short distances they have absorbed notable amounts of the stresses induced by the igneous intrusions. In such places not only the structure but the composition of the rock shows the effects of the intrusion. Certain shales in the section are carbonaceous and in all observed cases the organic matter has been transformed to anthracite, a condition generally associated with a certain amount of minute mashing and a cementation of both limestone and sandstone.

The granite becomes notably darker on approach to the northeastern contact near Colpani; the proportion of ferro-magnesian minerals in some cases is so large as to give a distinctly black color in sharp contrast to the nearly white granite typical of the central portion of the mass. Large masses of shale foundered in the invading magma, and upon fusion gave rise to huge black masses impregnated with quartz and in places smeared or injected with granite magma. Everywhere the granite is marked by numbers of black masses which appear at first sight to be aggregations of dark minerals normal to the granite and due to differentiation processes at the time of crystallization. It is, however, noteworthy that these increase rapidly in number on approach to the contact, until in the last half-mile they appear to grade into the shale inclusions. It may, therefore, be doubted that they are aggregations. From their universal distribution, their uniform character, and their marked increase in numbers on approach to lateral contacts, it may reasonably be inferred that they represent foundered masses of country rock. Those distant from present contacts are in almost all cases from a few inches to a foot in diameter, while on approach to lateral contacts they are in places ten to twenty feet in width, as if the smaller areas represented the last remnants of large inclusions engulfed in the magma near the upper or roof contact. They are so thoroughly injected with silica and also with typical granite magma as to make their reference to the country rock less secure on petrographical than on purely distributional grounds.

A parallel line of evidence relates to the distribution of complementary dikes throughout the granite. In the main mass of the batholith the dikes are rather evenly distributed as to kind with a slight preponderance of the dark-colored group. Near the contact, however, aplitic dikes cease altogether and great numbers of melanocratic dikes appear. It may be inferred that we have in this pronounced condition suggestions of strong influence upon the final processes of invasion and cooling of the granite magma, on the part of the country rock detached and absorbed by the invading mass. It might be supposed that the indicated change in the character of the complementary dikes could be ascribed to possible differentiation of the granite magma whereby a darker facies would be developed toward the Colpani contact. It has, however, been pointed out already that the darkening of the granite in this direction is intimately related to a marked increase in the number of inclusions, leaving little doubt that the thorough digestion of the smaller masses of detached shales is responsible for the marked increase in the number and variety of the ferro-magnesian and special contact minerals.

Upon the southwestern border of the batholith the number of aplitic dikes greatly increases. They form prominent features, not only of the granite, but also of the schists, adding greatly to the strong contrast between the schist of the border zone and that outside the zone of metamorphism. In places in the border schists, these are so numerous that one may count up to twenty in a single view, and they range in size from a few inches to ten or fifteen feet. The greater fissility of the schists as contrasted with the shales on the opposite or eastern margin of the batholith caused them to be relatively much more passive in relation to the granite magma. They were not so much torn off and incorporated in the magma, as they were thoroughly injected and metamorphosed. Added to this is the fact that they are petrographically more closely allied to the granite than are the shales upon the northeastern contact.