Part 2

In common with most rivers traversing the pre-Cambrian region, this water system is marked by a volume of dormant water enormously greater than that being transported at any given moment. With few exceptions the many small tributary brooks rise in lakes or groups of lakes surprisingly large in size, compared with the volume of the out-flowing streams; Otto and Lehmann lakes are drained by a rather sluggish rivulet 8 feet wide and 6ʺ deep, although their combined area is about two square miles. The larger streams themselves are only successions of irregular lake expansions which empty from one to another by short, river-like portions containing rapids and falls. The descent is therefore accomplished by a succession of abrupt steps rather than an evenly graded slope. This juvenile condition is directly ascribable to the geological character of the country; soil deposits are insignificant in quantity, leaving exposed a resistant and uneven rock floor in which the streams are unable to carve channels for themselves. Failing to do so they select the readiest egress by filling up impervious rock basins and spilling over at the lowest points into lower ones. In consequence of the scantiness of soils and frequency of natural settling basins, the waters of the whole system are free from suspended matter, and hence lack of an effective graving instrument. Exceptions to this general character occur in the extensive sand plain to the north and west of Duncan lake, where the several small creeks that traverse it are of ordinary fluvial form and gradation, and the waters of which transport large quantities of sand to Duncan lake.

FLORA AND FAUNA.

Where not recently burned the country is fairly well forested, the density and character of growth being dependent upon soil and drainage. The best timber is in low ground and near watercourses where conditions for growth are most favourable and the probability of fire least. Recent fires have done much damage around Nest, Obushkong and Gowganda lakes. Probably the best timber lies near Duncan lake. Trees do not grow very large as a rule and are not especially good for making lumber, but provide an abundant supply of materials for pulpwood, railway ties, fuel and for mine use.

White pine is the most valuable species, but although individuals attain thicknesses of 20ʺ to 40ʺ they are too scarce to render this timber worth the search. Good red pine is more abundant. Jack pine is a very common tree, especially in sandy districts, _e.g._, north-west of Duncan lake; but is small and worthless. The common and most widely distributed species are spruce, balsam, cedar, poplar, white and yellow birch. Tamarack is not abundant. Clumps of small red maples were seen to the south-west of Pigeon lake.

The East and West branches are not well suited for carrying logs as there are extensive lake expansions without current on each and the rapids are shallow. Excellent water-power is obtainable at the 40 foot fall on the West branch above Fort Matachewan.

Wild animals are not abundant, and will probably become less so as the country is occupied by prospecting and mining camps. Moose were plentiful in 1908 and some black bears were seen. As a source of food the abundance of pickerel and pike is of much greater importance. Brook trout do not occur in the Montreal River waters, but are caught farther to the west and north.

GENERAL GEOLOGY.

OUTLINE OF GEOLOGICAL HISTORY.

Though the geology of the region presents considerable complexity of detail, the general historical facts are distinct and go to show that the whole complex of formations and systems is capable of separation into four major divisions widely different from one another. The mutual relationships of these divisions, a knowledge of which is essential to a thorough comprehension of the geology, are succinctly expressed by the accompanying diagram.

The oldest division, the Keewatin, comprises a complex association of metamorphosed rocks, principally eruptive, characterized by well-developed, secondary schistosity and prevalently dark colours. They dip at angles approaching 90° and range in texture from soft, fissile, chorite schists to fine-grained gneisses or altered diabases. In the Montreal River district the Keewatin areas are not entirely visible, being overlain by other rocks, but they are thought to represent the bottoms of trough-like folds, produced by the upward intrusion of igneous matter which now constitutes the Laurentian. The latter forms the second division, its origin being apparent from the foregoing statement. It is wholly igneous, consisting of granite and allied coarsely crystalline rocks essentially pale-coloured owing to their richness in quartz and feldspars. Gneissic structure has been developed in varying degrees, so that all gradations between granite and gneisses exist; but it never attains the perfection found in the Keewatin. Near their contacts with the Keewatin, the gneisses are apt to contain dark bands and ribbons of the latter so highly crystalline as to conceal their identity.

Wherever visible the surface of the Keewatin and Laurentian presents an irregular, deeply worn appearance, the result of extremely protracted exposure to erosive agencies. To the best of geological knowledge the same conditions hold where they lie buried under the Huronian, indicating that a great period of denudation separates the latter from the Archæan. The combined Keewatin and Laurentian, or Archæan system, is therefore to be conceived as forming at all points in the district an ancient denuded foundation or floor upon which rests the much younger Huronian system.

This third division is, in the Montreal River district, wholly sedimentary and easily distinguished from the other rocks by its bedded structure and clastic nature. As it is the only sedimentary system represented, its members are not easily confused with any other, especially as their original structure is not obscured by metamorphic alteration. Locally this is not strictly true; in the vicinity of diabase intrusions they have been hardened and shattered so as to simulate the Keewatin, but the zones of alteration are narrow and readily identified by their gradation into adjacent areas of less altered types. At present the Huronian forms a discontinuous rock mantle over the Archæan, formerly more complete, but now worn through in places so as to expose portions of the crystalline basement.

The fourth division includes all eruptives known to be younger than the Huronian. Owing to the discontinuity of the latter it is not always easy to decide what rocks should be included in this group, for in some cases rather fresh-looking eruptives occur in the Keewatin which probably would also intrude the Huronian were it present; lacking the necessary information their chronological position can be only loosely fixed. By far the most extensive and important of the post-Huronian eruptives is the diabase with which the silver deposits are associated. This penetrates both Archæan and Huronian, but is ordinarily distinguishable by its unusual freshness, dark colour, and crystalline appearance. In certain cases, to be described subsequently, it may be confused with certain other diabases. Magmatic differentiative processes have evolved diabase types of very dissimilar appearance and mineralogical composition, of which a pink aplite occurring in dike form is the most extreme. Olivine diabase dikes are also present in the region, but in far less abundance.

Of little importance are the sands and gravels of glacial origin which lie thinly in the depressions and lower lands of the present glaciated surface.

TABLE OF FORMATIONS.

The geological events may be briefly enumerated in ascending order as follows:—

Deposits of glacial débris and weathering products of present surface.

Erosive period with glaciation.

Diabase intrusions.

Huronian sedimentation.

Erosion period.

Laurentian intrusion.

Keewatin.

KEEWATIN.

_General Features._

This system is considered as a complex assemblage of metamorphosed igneous rocks whose common and marked characteristics are pronounced alteration and deformation, accomplished in pre-Huronian times. A limited amount of sedimentary material, such as the iron ore formation, is also represented. These old diabases, porphyries and related types are much altered and have developed a more or less uniform schistosity through the secondary development of micaceous minerals, but in other respects the complex shows extreme inconstancy and variety from point to point. For this reason the various localities are separately described. By inspection of the map three fairly well defined areas are distinguishable, in addition to which are portions of several others.

_Obushkong area._

Much of the country between Obushkong and Firth lakes is underlain by Keewatin. On the east, south and west sides, these rocks disappear beneath the Huronian or are interrupted by masses of diabase, but on the north they merge into Laurentian gneiss, the contact with which is ill-defined. Although some of them retain much of their original massive character, well defined schistosity is the dominant feature. The schists stand vertically or at angles little less than 90° and trend in a general east and west direction. A series of compass observations made at points over the whole area show the strike to vary from N. 65° E. to S. 75° E.

One of the most abundant rock types is a stratiform, finely speckled hornblende gneiss or schist, the black hornblende cleavage faces giving it a glistening appearance on newly broken surfaces. It is quite fresh, perfectly crystalline and usually eminently fissile, but sometimes grades into a nearly massive dioritic form of undoubted igneous nature. Extensive exposures occur around Gould lake where the gneiss is traversed by numerous stringers of quartz, rusty in colour from the oxidation of pyrite. It is also well exposed near McLaughlin and McIntosh lakes and to the east and south-east of Foot lake. A fine grained chlorite schist of dull greenish black colour is common in this and all the other areas in the district. To the north-east of Serpentine lake it appears as a sheared phase of a weathered diabase, but it has also been derived from porphyry, exposures being seen between Foot and Obushkong lakes, where feldspar phenocrysts appear on weathered surfaces of the schist as pale, oblong spots. What is probably iron formation was observed at points 25 chains south of Gould lake, and 10 chains south of a little pond just east of Serpentine lake. Both outcrops consist of banded, grey quartzite interlaminated with chlorite schist, but magnetite-bearing bands were not found. Occasionally, narrow bands of pale grey, felsitic schists may be seen among the more common darker rocks. From evidence obtained at various points, these appear to have resulted from the decomposition and shearing of granite porphyry dikes probably connected with the Laurentian, and which penetrated the Keewatin during the time of Laurentian intrusion. Serpentine was observed between Foot lake and Obushkong at 20 chains from the latter. The surface is covered by a loose network of fine seams of asbestos which weather white and render the rock somewhat conspicuous. Its recognition is further simplified by the dull green, amorphous appearance of fresh surfaces, the slight translucence of thin edges and the glistening green seams of asbestos which traverse it abundantly. The same rock is more extensively exposed on the south-west of Serpentine lake and on Firth lake half a mile north of the portage leading to the former, also at less than a quarter of a mile south of this portage. It is associated with and derived from a dark green massive rock to which the name wehrlite is applicable, and a more detailed description of which appears later.

_Duncan Lake Area._

A Keewatin area of considerable extent lies between Duncan lake and the West branch in the vicinity of L’Africain and Beaverhouse lakes. Unlike the Obushkong area, the prevalent strike of the schists is nearly north and south, the greatest divergence noted being 25° W. In the former case Laurentian lies to the north, while in the present one it occurs on the east; in both cases the schistosity coincides approximately with the direction of the line of contact. On account of the swampy character of the country just west of the river and the scarcity of outcrops, this area was not completely explored, but wherever examined the Keewatin, as in the Obushkong area, consists predominantly of hornblende and chlorite schists, greenstone and decomposed diabase; but serpentine was not found. A nearly black diabase containing small grains of pyrite was observed 20 chains south-east of the southerly extending bay on Duncan lake, and outcrops of the same material were traced for about half a mile northward. Probably the same type was encountered just north of Beaverhouse lake and at some other points. Its unfoliated condition and fresher appearance than the adjacent rocks lead to the opinion that it is really post-Keewatin and intrusive, but the absence of younger rocks with which to correlate it, renders its exact age indeterminable. This rock exhibits enough resemblance to fractured contact edges of the post-Huronian diabase to make their distinction in the field rather difficult. At half a mile east of the south end of L’Africain lake is diabase which from lack of sufficient data has been mapped as Keewatin, although it may be identical with the post-Huronian variety. L’Africain lake lies in glistening hornblende gneiss with which are associated ribbons of a coarser hornblende gneiss belonging to the Laurentian of the area farther east. Greenstone and chlorite schists are the principal rocks around Beaverhouse lake. Associated with them and to the northward, are dikes of light coloured, granite porphyry, usually only a few feet wide, in some of which the original massive structure remains, while others show various gradations toward a felsitic, sericite-bearing schist. The isolated patch of Keewatin indicated on the map as occurring about half a mile east of the wide portion of Duncan lake, is a well foliated green schist in which oval white spots represent squeezed feldspar phenocrysts.

_Pigeon Lake Area._

West of Pigeon lake and the Montreal river, much of the country examined is underlain by Keewatin. Schistosity is less developed than in either of the preceding areas. The rocks are chiefly diabases and porphyry much decomposed and locally squeezed. Diabase occurs on both sides of Pigeon lake near its central islanded part and along the bay which extends south to Brush lake. An original diabase structure is sufficiently well preserved to show lath-like feldspars in hand specimens, but frequently the rock is altered to a chloritic mass. Around the bay extending toward Brush lake, this old diabase forms a rude wall intersected by a reticulating system of fractures filled with calcite, the resultant structure simulating that of a breccia. Shear zones seen on the east shore exhibit a pseudo-conglomeratic structure, the more resistant pieces of diabase having been partially rounded by the shearing movement and embedded in a matrix of finely pulverized rock matter. The same diabase apparently occurs all along the Montreal river where it follows the Algoma-Nipissing line. About half a mile above Pigeon lake it forms a 90 foot cliff on the west side of the river, near the top of which is a fresher looking, unfoliated eruptive. The form of this body was not ascertained, but microscopic examination of the specimen taken, determines it to be a hornblende lamprophyre, so probably it is a dike or thin sheet. The rock is fine-grained, dark green in colour and characterized by stout prisms of hornblende about one-quarter of an inch long, embedded in a finer, microcrystalline ground mass. Under the microscope it appears much decomposed. This rock was also observed near mile post 67 of the Algoma-Nipissing boundary, near the east end of the 63 chain portage leading to Breese lake and at other points, in all cases the exposures being of small extent.

In the last mentioned locality the predominant rock is an altered porphyry of ash grey colour. Around Porphyry lake it occurs in a fairly massive, easily recognizable condition, the feldspars showing as square white spots one-quarter of an inch in diameter in a grey, ground mass. On Breese lake it has been squeezed to a felsite schist, striking N. 20° W. In small amounts it is associated with post-Huronian diabase on the hills lying near the Montreal river and just south of the 63 chain portage.

The 300 foot hill south-west of Mosher lake is composed of a fine Keewatin greenstone whose surface is curiously weathered so as to suggest a spheroidal structure. It is marked off into round areas a foot or more in diameter by a sinuous network of weathered-out grooves. The main body of the rock is ordinary fine-grained greenstone, but the enclosing grooves have been formed in porphyritic zones about an inch in width. Among other types of less abundant distribution is an actinolite rock seen by the unassisted eye to consist of a felty mass of acicular crystals of actinolite, sometimes half an inch long. This rock was observed 12 chains east of the islanded part of Pigeon lake; also at points west of Montreal river below Pigeon lake.

_Unfinished Areas._

The Keewatin formation occupies the space between Near lake and the West branch, only part of which has been mapped. The rocks of this area are well foliated, standing as usual, vertically, and striking about N. 60° E. The exposures on the east shore of Near lake are entirely of fissile, chlorite schist, abundantly traversed by small barren quartz veins. Farther east the chlorite schist gives place to glistening, stratiform (laminated) hornblende gneiss with which are associated bands of a coarser hornblende gneiss, evidently the equivalent of granite. In one locality the stratiform rock contains subangular fragments of the coarser variety, quartz and a greyish eruptive rock, the resultant structure resembling that of a metamorphosed conglomerate or breccia. As some of the brecciated fragments occur near by in continuous bands within the stratiform gneiss, the clastic structure is to be ascribed to deformative movement instead of original sedimentation. A number of fine-grained quartz diabase dikes occur in the immediate vicinity.

A considerable portion of the east shore of Kenisheong lake consists of high, bare Keewatin hills, composed largely of chloritic schists, vertically inclined and striking east and west. Secondary calcite is richly disseminated, causing free effervescence when the rock is treated with acid. Pyrite is also abundant, sometimes segregated to form a lean ore, and at several points weathered superficially to limonite, colouring the cliffs dull red. Much-altered diabase like that on Pigeon lake is also present in subordinate amount.

Keewatin rocks also occur just east of Gowganda lake. A single brief visit was paid for the purpose of observing the iron formation which appears at this place, and further mention of which is made on a subsequent page.

LAURENTIAN.

_General Features._

Practically all the Laurentian lies in a continuous area, east of the West branch and extending beyond the limits of the map sheet. A smaller body occurs just west of Pigeon lake. The foliation in the Laurentian is less distinct than in the Keewatin and much less perfect. Its component rocks are mainly granite and allied plutonic types which pass by easy gradations into well-defined gneisses. Two principal granites are distinguishable; one containing black mica as its chief coloured constituent, the other hornblende. Both exhibit local variations in composition and crystalline texture, but are always pale grey rocks of ordinary granitic appearance.

The hornblende granite is a medium grained, fresh looking rock of speckled appearance, owing to the black hornblende crystals which lie scattered through the main mass of light grey feldspar and quartz. It is of uniform aspect, local variations of colour and texture being insignificant. As revealed under the microscope by a single thin section, it is an ordinary hornblende granite verging towards a syenite. Common green hornblende of idiomorphic prismatic habit, and usually twinned parallel to 100, is the principal ferromagnesian mineral. It is quite fresh, hence a few flakes of chlorite in the section were taken to represent an original small content of biotite; an acid oligoclase and orthoclase are the most abundant constituents; quartz is subordinate. Apatite, zircon and iron ore, probably magnetite, are accessory.

Hornblende granite is the commonest Laurentian rock in the neighbourhood of L’Africain and Sedge lakes, where it is in contact with the Keewatin. It also occurs on the East branch just below Obushkong lake. The area west of Pigeon lake seems to be composed wholly of this rock, outliers of which extend to Brush lake, and the islands and east shore of Pigeon lake. Here, however, it is distinctly syenitic, quartz being subordinate or absent. A distinctly porphyritic structure is apparent on Pigeon lake; the feldspars being well crystallized and lying in a finer grained, holocrystalline ground mass.

Biotite granite and gneiss are most prevalent in the northern and eastern portions of the area. No microscopic examination of these rocks has been made by the writer, consequently, little can be said regarding their composition. They are of much the same texture as the hornblende type, but show a somewhat higher degree of gneissification. Biotite is a fairly abundant constituent, but is more or less altered to chlorite. Sometimes, as may be seen on the East branch, two miles below Obushkong lake, chloritized mica forms enclosing films about the large feldspar grains, developing a slight ‘augen’ structure. Stockwork-like quartz veins are frequent, particularly on the West branch between Near and Sedge lakes and at the south end of Kenisheong lake. At the latter point the biotite gneiss is well foliated and steeply tilted. A body of deep red, biotite granite of undetermined extent and relationship was observed on the east side of Kenisheong lake. Pegmatite dikes almost certainly occur in this region, although not actually observed. Near Crotch lake the granite is locally of unusually coarse texture, although not truly pegmatitic.

Although the two granites described appear to be the essential constituents of the Laurentian they usually contain a variable proportion of other material, some of which at least is not really Laurentian. Over the entire area, but more noticeably in the vicinity of contacts with the Keewatin, they include narrow ribbons or lenses of a glistening stratiform hornblende gneiss, closely resembling the gneiss of this sort described under the Keewatin system. In some cases these bands are portions of the Keewatin caught up by the Laurentian material at the time of its intrusion and highly metamorphosed by it. Excellent examples of this may be seen on the East branch at the foot of the marshy stretch two miles below Obushkong lake. These inclusions are mapped as Laurentian, being an almost constant feature, and for map purposes inseparable from it. They are to be distinguished—a difficult matter—from other dark inclusions believed to be drawn out, basic segregations of Laurentian magmas; the latter are commonly less sharply defined.