Chapter 14

VOLCANOES

Connected with movements of the earth's crust which take place so slowly that they can be inferred only from their effects is one of the most rapid and impressive of all geological processes,--the extrusion of molten rock from beneath the surface of the earth, giving rise to all the various phenomena of volcanoes.

In a volcano, molten rock from a region deep below, which we may call its reservoir, ascends through a pipe or fissure to the surface. The materials erupted may be spread over vast areas, or, as is commonly the case, may accumulate about the opening, forming a conical pile known as the volcanic cone. It is to this cone that popular usage refers the word VOLCANO; but the cone is simply a conspicuous part of the volcanic mechanism whose still more important parts, the reservoir and the pipe, are hidden from view.

Volcanic eruptions are of two types,--EFFUSIVE eruptions, in which molten rock wells up from below and flows forth in streams of LAVA (a comprehensive term applied to all kinds of rock emitted from volcanoes in a molten state), and EXPLOSIVE eruptions, in which the rock is blown out in fragments great and small by the expansive force of steam.

ERUPTIONS OF THE EFFUSIVE TYPE

THE HAWAIIAN VOLCANOES. The Hawaiian Islands are all volcanic in origin, and have a linear arrangement characteristic of many volcanic groups in all parts of the world. They are strung along a northwest-southeast line, their volcanoes standing in two parallel rows as if reared along two adjacent lines of fracture or folding. In the northwestern islands the volcanoes have long been extinct and are worn low by erosion. In the southeastern island. Hawaii, three volcanoes are still active and in process of building. Of these Mauna Loa, the monarch of volcanoes, with a girth of two hundred miles and a height of nearly fourteen thousand feet above sea level, is a lava dome the slope of whose sides does not average more than five degrees. On the summit is an elliptical basin ten miles in circumference and several hundred feet deep. Concentric cracks surround the rim, and from time to time the basin is enlarged as great slices are detached from the vertical walls and engulfed.

Such a volcanic basin, formed by the insinking of the top of the cone, is called a CALDERA.

On the flanks of Mauna Loa, four thousand feet above sea level, lies the caldera of Kilauea, an independent volcano whose dome has been joined to the larger mountain by the gradual growth of the two. In each caldera the floor, which to the eye is a plain of black lava, is the congealed surface of a column of molten rock. At times of an eruption lakes of boiling lava appear which may be compared to air holes in a frozen river. Great waves surge up, lifting tons of the fiery liquid a score of feet in air, to fall back with a mighty plunge and roar, and occasionally the lava rises several hundred feet in fountains of dazzling brightness. The lava lakes may flood the floor of the basin, but in historic times have never been known to fill it and overflow the rim. Instead, the heavy column of lava breaks way through the sides of the mountain and discharges in streams which flow down the mountain slopes for a distance sometimes of as much as thirty-five miles. With the drawing off of the lava the column in the duct of the volcano lowers, and the floor of the caldera wholly or in part subsides. A black and steaming abyss marks the place of the lava lakes. After a time the lava rises in the duct, the floor is floated higher, and the boiling lakes reappear.

The eruptions of the Hawaiian volcanoes are thus of the effusive type. The column of lava rises, breaks through the side of the mountain, and discharges in lava streams. There are no explosions, and usually no earthquakes, or very slight ones, accompany the eruptions. The lava in the calderas boils because of escaping steam, but the vapor emitted is comparatively little, and seldom hangs above the summits in heavy clouds. We see here in its simplest form the most impressive and important fact in all volcanic action, molten rock has been driven upward to the surface from some deep-lying source.

LAVA FLOWS. As lava issues from the side of a volcano or overflows from the summit, it flows away in a glowing stream resembling molten iron drawn white-hot from an iron furnace. The surface of the stream soon cools and blackens, and the hard crust of nonconducting rock may grow thick and firm enough to form a tunnel, within which the fluid lava may flow far before it loses its heat to any marked degree. Such tunnels may at last be left as caves by the draining away of the lava, and are sometimes several miles in length.

PAHOEHOE AND AA. When the crust of highly fluid lava remains unbroken after its first freezing, it presents a smooth, hummocky, and ropy surface known by the Hawaiian term PAHOEHOE. On the other hand, the crust of a viscid flow may be broken and splintered as it is dragged along by the slowly moving mass beneath. The stream then appears as a field of stones clanking and grinding on, with here and there from some chink a dull red glow or a wisp of steam. It sets to a surface called AA, of broken, sharp-edged blocks, which is often both difficult and dangerous to traverse.

FISSURE ERUPTIONS. Some of the largest and most important outflows of lava have not been connected with volcanic cones, but have been discharged from fissures, flooding the country far and wide with molten rock. Sheet after sheet of molten rock has been successively outpoured, and there have been built up, layer upon layer, plateaus of lava thousands of feet in thickness and many thousands of square miles in area.

ICELAND. This island plateau has been rent from time to time by fissures from which floods of lava have outpoured. In some instances the lava discharges along the whole length of the fissure, but more often only at certain points upon it. The Laki fissure, twenty miles long, was in eruption in 1783 for seven months. The inundation of fluid rock which poured from it is the largest of historic record, reaching a distance of forty-seven miles and covering two hundred and twenty square miles to an average depth of a hundred feet. At the present time the fissure is traced by a line of several hundred insignificant mounds of fragmental materials which mark where the lava issued.

The distance to which the fissure eruptions of Iceland flow on slopes extremely gentle is noteworthy. One such stream is ninety miles in length, and another seventy miles long has a slope of little more than one half a degree.

Where lava is emitted at one point and flows to a less distance there is gradually built up a dome of the shape of an inverted saucer with an immense base but comparatively low. Many LAVA DOMES have been discovered in Iceland, although from their exceedingly gentle slopes, often but two or three degrees, they long escaped the notice of explorers.

The entire plateau of Iceland, a region as large as Ohio, is composed of volcanic products,--for the most part of successive sheets of lava whose total thickness falls little short of two miles. The lava sheets exposed to view were outpoured in open air and not beneath the sea; for peat bogs and old forest grounds are interbedded with them, and the fossil plants of these vegetable deposits prove that the plateau has long been building and is very ancient. On the steep sea cliffs of the island, where its structure is exhibited, the sheets of lava are seen to be cut with many DIKES,--fissures which have been filled by molten rock,--and there is little doubt that it was through these fissures that the lava outwelled in successive flows which spread far and wide over the country and gradually reared the enormous pile of the plateau.

ERUPTIONS OF THE EXPLOSIVE TYPE

In the majority of volcanoes the lava which rises in the pipe is at least in part blown into fragments with violent explosions and shot into the air together with vast quantities of water vapor and various gases. The finer particles into--which the lava is exploded are called VOLCANIC DUST or VOLCANIC ASHES, and are often carried long distances by the wind before they settle to the earth. The coarser fragments fall about the vent and there accumulate in a steep, conical, volcanic mountain. As successive explosions keep open the throat of the pipe, there remains on the summit a cup-shaped depression called the CRATER.

STROMBOLI. To study the nature of these explosions we may visit Stromboli, a low volcano built chiefly of fragmental materials, which rises from the sea off the north coast of Sicily and is in constant though moderate action.

Over the summit hangs a cloud of vapor which strikingly resembles the column of smoke puffed from the smokestack of a locomotive, in that it consists of globular masses, each the product of a distinct explosion. At night the cloud of vapor is lighted with a red glow at intervals of a few minutes, like the glow on the trail of smoke behind the locomotive when from time to time the fire bos is opened. Because of this intermittent light flashing thousands of feet above the sea, Stromboli has been given the name of the Lighthouse of the Mediterranean.

Looking down into the crater of the volcano, one sees a viscid lava slowly seething. The agitation gradually increases. A great bubble forms. It bursts with an explosion which causes the walls of the crater to quiver with a miniature earthquake, and an outrush of steam carries the fragments of the bubble aloft for a thousand feet to fall into the crater or on the mountain side about it. With the explosion the cooled and darkened crust of the lava is removed, and the light of the incandescent liquid beneath is reflected from the cloud of vapor which overhangs the cone.

At Stromboli we learn the lesson that the explosive force in volcanoes is that of steam. The lava in the pipe is permeated with it much as is a thick boiling porridge. The steam in boiling porridge is unable to escape freely and gathers into bubbles which in breaking spurt out drops of the pasty substance; in the same way the explosion of great bubbles of steam in the viscid lava shoots clots and fragments of it into the air.

KRAKATOA. The most violent eruption of history, that of Krakatoa, a small volcanic island in the strait between Sumatra and Java, occurred in the last week of August, 1883. Continuous explosions shot a column of steam and ashes. seventeen miles in air. A black cloud, beneath which was midnight darkness and from which fell a rain of ashes and stones, overspread the surrounding region to a distance of one hundred and fifty miles. Launched on the currents of the upper air, the dust was swiftly carried westward to long distances. Three days after the eruption it fell on the deck of a ship sixteen hundred miles away, and in thirteen days the finest impalpable powder from the volcano had floated round the globe. For many months the dust hung over Europe and America as a faint lofty haze illuminated at sunrise and sunset with brilliant crimson. In countries nearer the eruption, as in India and Africa, the haze for some time was so thick that it colored sun and moon with blue, green, and copper-red tints and encircled them with coronas.

At a distance of even a thousand miles the detonations of the eruption sounded like the booming of heavy guns a few miles away. In one direction they were audible for a distance as great as that from San Francisco to Cleveland. The entire atmosphere was thrown into undulations under which all barometers rose and fell as the air waves thrice encircled the earth. The shock of the explosions raised sea waves which swept round the adjacent shores at a height of more than fifty feet, and which were perceptible halfway around the globe.

At the close of the eruption it was found that half the mountain had been blown away, and that where the central part of the island had been the sea was a thousand feet deep.

MARTINIQUE AND ST. VINCENT. In 1902 two dormant volcanoes of the West Indies, Mt. Pelee in Martinique and Soufriere in St. Vincent, broke into eruption simultaneously. No lava was emitted, but there were blown into the air great quantities of ashes, which mantled the adjacent parts of the islands with a pall as of gray snow. In early stages of the eruption lakes which occupied old craters were discharged and swept down the ash-covered mountain valleys in torrents of boiling mud.

On several occasions there was shot from the crater of each volcano a thick and heavy cloud of incandescent ashes and steam, which rushed down the mountain side like an avalanche, red with glowing stones and scintillating with lightning flashes. Forests and buildings in its path were leveled as by a tornado, wood was charred and set on fire by the incandescent fragments, all vegetation was destroyed, and to breathe the steam and hot, suffocating dust of the cloud was death to every living creature. On the morning of the 8th of May, 1902, the first of these peculiar avalanches from Mt. Pelee fell on the city of St. Pierre and instantly destroyed the lives of its thirty thousand inhabitants.

The eruptions of many volcanoes partake of both the effusive and the explosive types: the molten rock in the pipe is in part blown into the air with explosions of steam, and in part is discharged in streams of lava over the lip of the crater and from fissures in the sides of the cone. Such are the eruptions of Vesuvius, one of which is illustrated in Figure 219.

SUBMARINE ERUPTIONS. The many volcanic islands of the ocean and the coral islands resting on submerged volcanic peaks prove that eruptions have often taken place upon the ocean floor and have there built up enormous piles of volcanic fragments and lava. The Hawaiian volcanoes rise from a depth of eighteen thousand feet of water and lift their heads to about thirty thousand feet above the ocean bed. Christmas Island (see p. 194), built wholly beneath the ocean, is a coral-capped volcanic peak, whose total height, as measured from the bottom of the sea, is more than fifteen thousand feet. Deep-sea soundings have revealed the presence of numerous peaks which fail to reach sea level and which no doubt are submarine volcanoes. A number of volcanoes on the land were submarine in their early stages, as, for example, the vast pile of Etna, the celebrated Sicilian volcano, which rests on stratified volcanic fragments containing marine shells now uplifted from the sea.

Submarine outflows of lava and deposits of volcanic fragments become covered with sediments during the long intervals between eruptions. Such volcanic deposits are said to be CONTEMPORANEOUS, because they are formed during the same period as the strata among which they are imbedded. Contemporaneous lava sheets may be expected to bake the surface of the stratum on which they rest, while the sediments deposited upon them are unaltered by their heat. They are among the most permanent records of volcanic action, far outlasting the greatest volcanic mountains built in open air.

From upraised submarine volcanoes, such as Christmas Island, it is learned that lava flows which are poured out upon the bottom of the sea do not differ materially either in composition or texture from those of the land.

VOLCANIC PRODUCTS

Vast amounts of steam are, as we have seen, emitted from volcanoes, and comparatively small quantities of other vapors, such as various acid and sulphurous gases. The rocks erupted from volcanoes differ widely in chemical composition and in texture.

ACIDIC AND BASIC LAVAS. Two classes of volcanic rocks may be distinguished,--those containing a large proportion of silica (silicic acid, SiO2) and therefore called ACIDIC, and those containing less silica and a larger proportion of the bases (lime, magnesia, soda, etc.) and therefore called BASIC. The acidic lavas, of which RHYOLITE and THRACHYTE are examples, are comparatively light in color and weight, and are difficult to melt. The basic lavas, of which BASALT is a type, are dark and heavy and melt at a lower temperature.

SCORIA AND PUMICE. The texture of volcanic rocks depends in part on the degree to which they were distended by the steam which permeated them when in a molten state. They harden into compact rock where the steam cannot expand. Where the steam is released from pressure, as on the surface of a lava stream, it forms bubbles (steam blebs) of various sizes, which give the hardened rock a cellular structure (Fig. 220), In this way are formed the rough slags and clinkers called SCORIA, which are found on the surface of flows and which are also thrown out as clots of lava in explosive eruptions.

On the surface of the seething lava in the throat of the volcano there gathers a rock foam, which, when hurled into the air, is cooled and falls as PUMICE,--a spongy gray rock so light that it floats on water.

AMYGDULES. The steam blebs of lava flows are often drawn out from a spherical to an elliptical form resembling that of an almond, and after the rock has cooled these cavities are gradually filled with minerals deposited from solution by underground water. From their shape such casts are called amygdules (Greek, amygdalon, an almond). Amygdules are commonly composed of silica. Lavas contain both silica and the alkalies, potash and soda, and after dissolving the alkalies, percolating water is able to take silica also into solution. Most AGATES are banded amygdules in which the silica has been laid in varicolored, concentric layers.

GLASSY AND STONY LAVAS. Volcanic rocks differ in texture according also to the rate at which they have solidified. When rapidly cooled, as on the surface of a lava flow, molten rock chills to a glass, because the minerals of which it is composed have not had time to separate themselves from the fused mixture and form crystals. Under slow cooling, as in the interior of the flow, it becomes a stony mass composed of crystals set in a glassy paste. In thin slices of volcanic glass one may see under the microscope the beginnings of crystal growth in filaments and needles and feathery forms, which are the rudiments of the crystals of various minerals.

Spherulites, which also mark the first changes of glassy lavas toward a stony condition, are little balls within the rock, varying from microscopic size to several inches in diameter, and made up of radiating fibers.

Perlitic structure, common among glassy lavas, consists of microscopic curving and interlacing cracks, due to contraction.

FLOW LINES are exhibited by volcanic rocks both to the naked eye and under the microscope. Steam blebs, together with crystals and their embryonic forms, are left arranged in lines and streaks by the currents of the flowing lava as it stiffened into rock.

PORPHYRITIC STRUCTURE. Rocks whose ground mass has scattered through it large conspicuous crystals are said to be PORPHYRITIC, and it is especially among volcanic rocks that this structure occurs. The ground mass of porphyries either may be glassy or may consist in part of a felt of minute crystals; in either case it represents the consolidation of the rock after its outpouring upon the surface. On the other hand, the large crystals of porphyry have slowly formed deep below the ground at an earlier date.

COLUMNAR STRUCTURE. Just as wet starch contracts on drying to prismatic forms, so lava often contracts on cooling to a mass of close-set, prismatic, and commonly six-sided columns, which stand at right angles to the cooling surface. The upper portion of a flow, on rapid cooling from the surface exposed to the air, may contract to a confused mass of small and irregular prisms; while the remainder forms large and beautifully regular columns, which have grown upward by slow cooling from beneath.

FRAGMENTAL MATERIALS

Rocks weighing many tons are often thrown from a volcano at the beginning of an outburst by the breaking up of the solidofied floor of the crater; and during the progress of an eruption large blocks may be torn from the throat of the volcano by the outrush of steam. But the most important fragmental materials are those derived from the lava itself. As lava rises in the pipe, the steam which permeates it is released from pressure and explodes, hurling the lava into the air in fragments of all sizes,--large pieces of scoria, LAPILLI (fragments the size of a pea or walnut), volcanic "sand" and volcanic "ashes." The latter resemble in appearance the ashes of wood or coal, but they are not in any sense, like them, a residue after combustion.

Volcanic ashes are produced in several ways: lava rising in the volcanic duct is exploded into fine dust by the steam which permeates it; glassy lava, hurled into the air and cooled suddenly, is brought into a state of high strain and tension, and, like Prince Rupert's drops, flies to pieces at the least provocation. The clash of rising and falling projectiles also produces some dust, a fair sample of which may be made by grating together two pieces of pumice.

Beds of volcanic ash occur widely among recent deposits in the western United States. In Nebraska ash beds are found in twenty counties, and are often as white as powdered pumice. The beds grow thicker and coarser toward the southwestern part of the state, where their thickness sometimes reaches fifty feet. In what direction would you look for the now extinct volcano whose explosive eruptions are thus recorded?

TUFF. This is a convenient term designating any rock composed of volcanic fragments. Coarse tuffs of angular fragments are called VOLCANIC BRECIA, and when the fragments have been rounded and sorted by water the rock is termed a VOLCANIC CONGLOMERATE. Even when deposited in the open air, as on the slopes of a volcano, tuffs may be rudely bedded and their fragments more or less rounded, and unless marine shells or the remains of land plants and animals are found as fossils in them, there is often considerable difficulty in telling whether they were laid in water or in air. In either case they soon become consolidated. Chemical deposits from percolating waters fill the interstices, and the bed of loose fragments is cemented to hard rock.

The materials of which tuffs are composed are easily recognized as volcanic in their origin. The fragments are more or less cellular, according to the degree to which they were distended with steam when in a molten state, and even in the finest dust one may see the glass or the crystals of lava from which it was derived. Tuffs often contain VOCLANIC BOMBS,--balls of lava which took shape while whirling in the air, and solidified before falling to the ground.

ANCIENT VOLCANIC ROCKS. It is in these materials and structures which we have described that volcanoes leave some of their most enduring records. Even the volcanic rocks of the earliest geological ages, uplifted after long burial beneath the sea and exposed to view by deep erosion, are recognized and their history read despite the many changes which they may have undergone. A sheet of ancient lava may be distinguished by its composition from the sediments among which it is imbedded. The direction of its flow lines may be noted. The cellular and slaggy surface where the pasty lava was distended by escaping steam is recognized by the amygdules which now fill the ancient steam blebs. In a pile of successive sheets of lava each flow may be distinguished and its thickness measured; for the surface of each sheet is glassy and scoriaceous, while beneath its upper portions the lava of each flow is more dense and stony. The length of time which elapsed before a sheet was buried beneath the materials of succeeding eruptions may be told by the amount of weathering which it had undergone, the depth of ancient soil--now baked to solid rock--upon it, and the erosion which it had suffered in the interval.

If the flow occurred from some submarine volcano, we may recognize the fact by the sea-laid sediments which cover it, filling the cracks and crevices of its upper surface and containing pieces of lava washed from it in their basal layers.

Long-buried glassy lavas devitrify, or pass to a stony condition, under the unceasing action of underground waters; but their flow lines and perlitic and spherulitic structures remain to tell of their original state.

Ancient tuffs are known by the fragmental character of their volcanic material, even though they have been altered to firm rock. Some remains of land animals and plants may be found imbedded to tell that the beds were laid in open air; while the remains of marine organisms would prove as surely that the tuffs were deposited in the sea.

In these ways ancient volcanoes have been recognized near Boston, in southeastern Pennsylvania, about Lake Superior, and in other regions of the United States.

THE LIFE HISTORY OF A VOLCANO

The invasion of a region by volcanic forces is attended by movements of the crust heralded by earthquakes. A fissure or a pipe is opened and the building of the cone or the spreading of wide lava sheets is begun.

VOLCANIC CONES. The shape of a volcanic cone depends chiefly on the materials erupted. Cones made of fragments may have sides as steep as the angle of repose, which in the case of coarse scoria is sometimes as high as thirty or forty degrees. About the base of the mountain the finer materials erupted are spread in more gentle slopes, and are also washed forward by rains and streams. The normal profile is thus a symmetric cone with a flaring base.

Cones built of lava vary in form according to the liquidity of the lava. Domes of gentle slope, as those of Hawaii, for example, are formed of basalt, which flows to long distances before it congeals. When superheated and emitted from many vents, this easily melted lava builds great plateaus, such as that of Iceland. On the other hand, lavas less fusible, or poured out at a lower temperature, stiffen when they have flowed but a short distance, and accumulate in a steep cone. Trachyte has been extruded in a state so viscid that it has formed steepsided domes like that of Sarcoui.

Most volcanoes are built, like Vesuvius, both of lava flows and of tuffs, and sections show that the structure of the cone consists of outward-dipping, alternating layers of lava, scoria, and ashes.

From time to time the cone is rent by the violence of explosions and by the weight of the column of lava in the pipe. The fissures are filled with lava and some discharge on the sides of the mountain, building parasitic cones, while all form dikes, which strengthen the pile with ribs of hard rock and make it more difficult to rend.

Great catastrophes are recorded in the shape of some volcanoes which consist of a circular rim perhaps miles in diameter, inclosing a vast crater or a caldera within which small cones may rise. We may infer that at some time the top of the mountain has been blown off, or has collapsed and been engulfed because some reservoir beneath had been emptied by long-continued eruptions.

The cone-building stage may be said to continue until eruptions of lava and fragmental materials cease altogether. Sooner or later the volcanic forces shift or die away, and no further eruptions add to the pile or replace its losses by erosion during periods of repose. Gases however are still emitted, and, as sulphur vapors are conspicuous among them, such vents are called SOLFATARAS. Mount Hood, in Oregon, is an example of a volcano sunk to this stage. From a steaming rift on its side there rise sulphurous fumes which, half a mile down the wind, will tarnish a silver coin.

GEYSERS AND HOT SPRINGS. The hot springs of volcanic regions are among the last vestiges of volcanic heat. Periodically eruptive boiling springs are termed geysers. In each of the geyser regions of the earth--the Yellowstone National Park, Iceland, and New Zealand--the ground water of the locality is supposed to be heated by ancient lavas that, because of the poor conductivity of the rock, still remain hot beneath the surface.

OLD FAITHFUL, one of the many geysers of the Yellowstone National Park, plays a fountain of boiling water a hundred feet in air; while clouds of vapor from the escaping steam ascend to several times that height. The eruptions take place at intervals of from seventy to ninety minutes. In repose the geyser is a quiet pool, occupying a craterlike depression in a conical mound some twelve feet high. The conduit of the spring is too irregular to be sounded. The mound is composed of porous silica deposited by the waters of the geyser.

Geysers erupt at intervals instead of continuously boiling, because their long, narrow, and often tortuous conduits do not permit a free circulation of the water. After an eruption the tube is refilled and the water again gradually becomes heated. Deep in the tube where it is in contact with hot lavas the water sooner or later reaches the boiling point, and bursting into steam shoots the water above it high in air.

CARBONATED SPRINGS. After all the other signs of life have gone, the ancient volcano may emit carbon dioxide as its dying breath. The springs of the region may long be charged with carbon dioxide, or carbonated, and where they rise through limestone may be expected to deposit large quantities of travertine. We should remember, however, that many carbonated springs, and many hot springs, are wholly independent of volcanoes.

THE DESTRUCTION OF THE CONE. As soon as the volcanic cone ceases to grow by eruptions the agents of erosion begin to wear it down, and the length of time that has elapsed since the period of active growth may be roughly measured by the degree to which the cone has been dissected. We infer that Mount Shasta, whose conical shape is still preserved despite the gullies one thousand feet deep which trench its sides, is younger than Mount Hood, which erosive agencies have carved to a pyramidal form. The pile of materials accumulated about a volcanic vent, no matter how vast in bulk, is at last swept entirely away. The cone of the volcano, active or extinct, is not old as the earth counts time; volcanoes are short- lived geological phenomena.

CRANDALL VOLCANO. This name is given to a dissected ancient volcano in the Yellowstone National Park, which once, it is estimated, reared its head thousands of feet above the surrounding country and greatly exceeded in bulk either Mount Shasta or Mount Etna. Not a line of the original mountain remains; all has been swept away by erosion except some four thousand feet of the base of the pile. This basal wreck now appears as a rugged region about thirty miles in diameter, trenched by deep valleys and cut into sharp peaks and precipitous ridges. In the center of the area is found the nucleus (N, Fig. 237),--a mass of coarsely crystalline rock that congealed deep in the old volcanic pipe. From it there radiate in all directions, like the spokes of a wheel, long dikes whose rock grows rapidly finer of grain as it leaves the vicinity of the once heated core. The remainder of the base of the ancient mountain is made of rudely bedded tuffs and volcanic breccia, with occasional flows of lava, some of the fragments of the breccia measuring as much as twenty feet in diameter. On the sides of canyons the breccia is carved by rain erosion to fantastic pinnacles. At different levels in the midst of these beds of tuff and lava are many old forest grounds. The stumps and trunks of the trees, now turned to stone, still in many cases stand upright where once they grew on the slopes of the mountain as it was building (Fig. 238). The great size and age of some of these trees indicate, the lapse of time between the eruption whose lavas or tuffs weathered to the soil on which they grew and the subsequent eruption which buried them beneath showers of stones and ashes.

Near the edge of the area lies Death Gulch, in which carbon dioxide is given off in such quantities that in quiet weather it accumulates in a heavy layer along the ground and suffocates the animals which may enter it.