HURONIAN.
General Features.
Much of the area mapped, especially the north-western portion, is composed of Huronian rocks. Originally they must have been much more extensive, probably continuous, but erosion has removed them partially or entirely, leaving irregular remnants distributed over the Archæan. The top of the series is gone; consequently complete vertical sections cannot be found. In other respects they are little changed and preserve almost perfectly their bedding and clastic structure. Even their positions assumed at the time of deposition have changed little for the present strata seldom dip more steeply than 30°. They are overlain only by unconsolidated Pleistocene materials.
Basal Conglomerate.
The basal member of this system is a conglomerate, the areal exposure of which is inconsiderable for the whole region or for any part, but erosional sculpturing has brought it to light at many points. Both top and bottom and probably all intermediate portions are visible, but a complete section from which to determine the thickness could not be found. The greatest continuous vertical section observed, in a hill lying one mile south of the 550 foot hill north-west of Duncan lake, is about 200 feet, but neither upper nor lower terminations were seen. In the neighbouring Cobalt district an estimated thickness of 500 feet has been assigned.
From a study of the pebbles contained, this conglomerate appears to be composed largely, or entirely, of Archæan materials, the majority of which are rock types occurring in situ at many places in the district. Both the hornblende and biotite granite and their gneissic phases are abundant. Pegmatite pebbles are sometimes found, but more usually that rock is represented by fragments of feldspar. Pieces of glistening stratiform hornblende gneiss and similar stratiform mica gneiss or schist, greenstone, fine-grained porphyroids and sheared basic rocks represent the Keewatin. Vein quartz is also present, sometimes mineralized. In addition to these some pebbles were observed which closely resembled slate, and one which is undoubtedly conglomeratic. A thin section of this pebble exhibits distinct clastic structure, and the assembled fragments are heterogeneous, so far as distinguishable, including an actinolite rock resembling that found in the Keewatin near Pigeon lake, and a quartzitic type composed of a colourless, microcrystalline mosaic. The cement has been altered largely to chlorite, but small grains of quartz are discernible in it. It would seem, therefore, that sedimentary deposits antedate the basal conglomerate and supply a portion of its materials; most of these, however, are recognizable as igneous members of the Archæan, such as occur in the vicinity.
The pebbles of the conglomerate exhibit remarkable variety in form, size and abundance, as well as composition. Normally they are well rounded and owe their form evidently to water action. But along with these are others which are angular or sub-angular. The exposure near the north end of Shallow lake shows all these forms, the materials also being quite diverse. Ordinarily the pebbles range in size between 2ʺ and 3ʺ diameter, frequently more, and are abundant; but, at points on Duncan lake, on Wapus creek and the West branch, the conglomerate nature is indicated only by occasional well-rounded pebbles, embedded in greywacke cement at intervals of several feet or even yards. These isolated pebbles are often 6ʺ or more in diameter, and in striking contrast with the uniformly fine grain of the matrix. Sometimes the enclosed bodies attain the dimensions of boulders; on an island in Duncan lake, 21⁄2 miles from the foot, a granite boulder nearly 5 feet in diameter was observed, the associated materials being of very much smaller size.
The cementing material also presents some variety. Usually it ranges from coarse grit to greywacke. The coarser material is less abundant than cement of a finer type; near the foot of Pigeon lake, also at the south end of Kenisheong lake the conglomerate is quite porous, the interstices between the pebbles being incompletely filled. In the latter instance these spaces are occupied by hematite. In other cases; a good example of which occurs on the east side of Pigeon lake, half a mile from its north end, the cementing substance is a fine black shale. On the whole there appears to be little relationship between the texture of enclosing and enclosed matter.
Greywacke Slate and Quartzite.
By the disappearance of pebbles the basal conglomerate changes into a greywacke or a shale according as the cement is one or the other. Frequently this transition is marked by an alternation of lenticular beds of conglomerate with the greywacke, probably indicating varying conditions of depth or current in the water in which they were deposited. With the greywacke and shale is associated an impure quartzite or arkose, the whole forming a thick series whose members are not sharply separable one from another and do not occupy definite relative positions. For the well laminated finer grained beds the term slate is in general use, although a secondary cleavage by which this kind of rock is distinguished from shale does not exist. Nearly all the prominent hills in the north-western part of the area are composed of this series.
Upper Conglomerate.
The greywacke-slate-quartzite series passes conformably upward into a conglomerate differing little from that at the base. Indeed, where conglomerate outcrops are small and isolated a distinction between the two cannot be made with certainty.
Arkose.
In addition to the above there is an arkose forming at least two well defined areas, whose relations with the rest of the Huronian are in some doubt. This rock is of distinctive appearance, resembling at first glance an ordinary granite, but on closer inspection it is seen to consist of clastic materials such as would result from the disintegration of a granite. Occasional conglomerate streaks in which pebbles of quartz and greenstone are recognizable, demonstrate its bedded character, but ordinarily it is massive looking. Part of the large island in the middle of Duncan lake and the shore to the south are of this formation. It is much more widespread on Obushkong and Gowganda lakes, the bedding being unusually well shown on the large island in the latter body of water.
From its apparent relationship in either of these localities it might be taken to be a member of the Lower Huronian series just described, and equivalent to the basal conglomerate. In the Obushkong area it is probably underlain by Laurentian, for it lies nearly horizontally, and Laurentian is known to occur a short distance east of the lake. In the neighbourhood of Lake Timiskaming it is said to grade imperceptibly into granite, and is believed to be derived from the latter by detrition in situ. Arkose, apparently identical with that under consideration, occurs in the Cobalt district, and is thought to lie unconformably with the greywacke, and for this reason is classified as Middle Huronian. In these pages it is given no definite position in the formational succession, and as there is no field evidence of its Middle Huronian position, is not differentiated from the lower series.
Structural Features and Disturbances.
The Huronian has been subjected to no very severe disturbance, judging by its present condition and attitude. Frequently the strata lie almost horizontally, as for instance at the south end of Firth lake, and in the vicinity of Lake Lehmann, but over the most of the region they rest at inclinations as high as 30°, this attitude remaining constant over extensive areas, and developing a characteristic topographical feature. From Pigeon lake eastward the dip is uniformly to the east at angles ranging between 15 and 30 degrees and the beds overlap one another after the fashion of slates on a roof, the resultant topographical expression of which is a succession of north and south ridges with gentle eastern slopes, while the western sides form escarpments. This condition appears constant over all the Huronian east of a line midway between Pigeon and Duncan lakes. To the extreme north-west, however, a westerly dip was observed; the strata forming the 550 foot hill west of Duncan lake are also either horizontal or dip gently to the west. A confident statement cannot be made until further work has been performed, but the condition just outlined suggests a large anticlinal structure whose arch lies a little west of Duncan lake. However, there seems reason to believe that the structure is more complicated than would result from simple arching and erosion. If the present overlapping system represents the original succession of strata a total thickness of over two miles would be necessary, and there would be not two but several conglomerate horizons, which is improbable. A satisfactory solution is hindered by the general fact that planes of possible dislocation occur in low ground and are obscured by swamp or water, but it seems evident that tilting was accompanied by lateral or vertical displacements.
Certain abrupt disturbances of the general uniform attitude suggest differential movements. Along the West branch below Wapus creek the dip and strike of the shale and greywacke are constant, until where the river’s course changes to due east. Along the shores of this stretch the rocks are mostly hidden by swamp, but where they do outcrop they are standing vertically or dipping steeply to the south, and the strike corresponds with the course of the stream; that is, their positions are at right angles to those farther south. Disturbances of this sort are known, due to the contiguity of igneous intrusions, but at this point no such intrusive is known. It will also be seen from the map that exactly in the same line the East branch makes a similar abrupt change in course. Whether a line of low relief is continuous across the interval between the two streams at this point is not easily determined; however, as a possible explanation of the conditions stated, faulting along this line is suggested. A similar abrupt change from conditions of approximate horizontality to a dip S. 60 W., < 80° and strike of S. 30° E. was noted on the west side of Firth lake.
Relations to other Formations.
The intrusion of the quartz diabase into the Huronian was gently accomplished at most points, and the beds of the latter, both above and below the intrusives, are inclined only a few degrees more steeply than in localities where no diabase can be found. Evidence of intrusion, however, is common, and at some points the Huronian next to the diabase has suffered local but intense physical change. Near the middle of the east shore of Firth lake a rocky islet only a few square yards in extent consists of coarse diabase and conglomerate in intimate contact, little tongues of the former being protruded into the sedimentary rock and peripherally chilled. Most remarkable, however, is the change in condition of the conglomerate; a few chains away on the main shore it is of ordinary character, but on the islet the pebbles lie within a fairly well foliated schist, standing vertically and striking about east and west. An identical condition exists at the south end of a little pond lying 40 chains west of Mosher lake. The vertical foliation of the conglomerate suggests the neighbouring diabase intrusion to have been by vertical ascension rather than lateral spread, so that these points may represent portions of vents through which the diabase magma ascended, and for that reason are more affected than where sills have been quietly injected.
Ordinarily these contacts occur in low ground, usually ravines, the bottoms of which are soil filled and consequently unfavourable for geological observation, but at some points on Duncan lake the contacts are exposed and the Huronian is seen to be much fractured across a zone extending many feet from the diabase. In addition to the fracturing the greywacke is hardened and the bedding planes rendered obscure, the total effect being to weaken its power of resistance to erosion.
In the neighbourhood of these contacts the Huronian is well supplied with quartz veins whose ‘comb’ structure and chalcopyrite-galena mineralization identify them with similar veins in the diabase. Also the sediments exhibit certain mineralogical alterations referable to action of the diabase. West of Gowganda, on the West branch below Duncan lake and at other places the greywacke for a width of about two feet from the diabase has been hardened, bleached a light grey colour and filled with circular black spots 1⁄16ʺ in diameter. Microscopic examination shows the main portion to consist of a mosaic of small quartz, orthoclase and acid plagioclase grains, through which are distributed patches of chlorite. This spotted phase of the greywacke evidently represents an early stage in the development of adinole, a characteristic contact product of shales, intruded by basic rock.
Fig. 6. Fifth Portage on the West Branch of Montreal River, showing Huronian Slate.
The relations of the Huronian to the Archæan, exhibited in larger as well as minor features, indicate that the surface of the latter had been carved into a condition not greatly unlike the present topography, before its submergence and sedimentation in Huronian times. Some of the hills of this ancient land have been uncovered by the removal of their sedimentary blanket, leaving vestiges of the latter around their bases as in the case of the hills south-west of Mosher lake. There the dip of the sedimentary beds is much less than the slope of the hills, so that the latter must project up through them as cores. In the bay on the east side of Pigeon lake Huronian shales dipping with an angle of about 15° abut against the side of an Archæan hill composed of hornblende granite and green schist. At almost any part of the region where both Huronian and Archæan occur together, similar evidence is available concerning the unevenness of the pre-Huronian land surface. The amount of topographical relief cannot be estimated with any degree of accuracy, for tops of the Keewatin hills now exposed have probably been removed by post-Huronian erosion, the lowest depressions are still filled by Huronian and the whole may have been disarranged by post-Archæan faulting. However, near Mosher lake there was a minimum relief of 300 feet.
This old pre-Huronian surface is not often accessible for study, the contacts being vertical or hidden by soils and vegetation. At one point, however, at the end of the portion of Wapus creek shown on the map, exceptionally favourable circumstances were encountered. Here glaciation has developed a rounded knoll of mixed Keewatin and Laurentian rocks upon which are tightly fastened a few scale-like vestiges of Huronian conglomerate. A few square feet of the original Archæan surface exposed by chiseling away the Huronian, was found to be much more highly polished than the immediately adjoining, recently glaciated surface which had been exposed to the atmosphere. The protected material seemed to be of about the same freshness as that exposed. This pre-Huronian surface is evidently a water-worn one, or the result of Huronian glaciation, but no characteristic markings were observed upon it.
Origin and Correlation.
From the foregoing consideration of the Huronian as seen in the Montreal River region it will be plain that it is composed at all points of clastic sedimentary deposits. Fossils have never been found, so the correlation of these rocks in various areas is based upon their lithological similarities and continuity. Upon these grounds the Huronian of the present district is considered to be equivalent to the same formations in the Cobalt, Larder Lake, and other neighbouring districts, known as the Lower Huronian. The succession and physical character of the different formations are essentially identical. It is not necessary to review the facts upon which the decision to so place these formations is based, the evidence being the harmonious results of years of investigation by capable geologists. Accepting their conclusion, and calling the main sedimentary series of the Montreal River district, Lower Huronian, it is of interest to note indication of sedimentary materials still older. The conglomerate pebble found in the basal conglomerate on Pigeon lake must have originated by the destruction of a pre-Huronian conglomerate formation.
It has also been stated that ground for differentiating between a Lower and Middle Huronian in this area has not been obtained, although the similarity of the arkose to a formation in the Cobalt district believed to belong to the Middle Huronian suggests it to be of the same age. It is, therefore, thought desirable to apply to the whole the term Huronian. No clearness or additional truth would be gained by making a distinction between Lower and Middle divisions in this district, and so far as the economic exploitation of the district is concerned the arkose seems to be as much a part of the Lower Huronian as the conglomerate or greywacke.