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, 2-1/2
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.
[Illustration: 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.
POST-HURONIAN ERUPTIVES.
There are two kinds of diabase known in the district, one of which
greatly exceeds the other in extent and economic importance. This
disparity is tacitly recognized by the common use of the generic term
diabase for the important quartz diabase, the other member of the family
being ignored or unknown.
_Quartz Diabase._
[Illustration: Fig. 7. Inclined Huronian Beds, Duncan Lake.]
_Character of contacts and distribution._—Bodies of this rock are not
restricted to any one formation or locality; but, as may be seen by
reference to the map, occur with various dimensions over the whole area,
with the exception of the country lying north-west of Duncan lake. It
may be noted that virtually all the large areas are enclosed by
Huronian, the bodies within the Archæan being numerous but small. It is
not proposed to apply this distinction generally, but in the Montreal
River region it seems to be more than an expression of the law of
probability. In the Archæan practically all of the observed diabase
bodies are dikes, in approximately vertical positions, seldom 100 feet
in width and of undetermined length. A knowledge of all those in the
Huronian could not be obtained, but in some cases they are sills, whose
greater exposed dimensions are parallel to the bedding planes of the
enclosing sedimentaries.
_Macroscopic character._—It is fortunate for the easy recognition of the
diabase that it is more or less continuously exposed, and that there are
not many other igneous rocks of similar character in its proximity, for
it presents a variety of types such that a representative collection of
hand specimens presents surprisingly great petrological differences.
During the process of solidification, magmatic differentiation evolved
from the cooling material a group of forms of different mineralogical
composition and physical appearance, the end members of which are very
unlike.
The common type is a dark green massive diabase, ranging in texture from
one in which the individual minerals are barely distinguishable to
others containing amphibole crystals an inch in length. The combination
of black amphibole—an alteration product of augite—and grey or
flesh-coloured plagioclase give the surface of ordinary textured phases
a colouration of sufficient determinative value. In a large diabase body
the coarseness is equal to that of ordinary granite, and the diabasic
structure is not readily perceptible. At the edge of the large dikes and
throughout the smaller ones the rock is black in colour and much finer
grained on account of its more rapid solidification.
In the dikes the mineralogical composition is tolerably constant, and
specimens selected from different points show no notable difference
except that they are usually less decomposed than the coarse grained
varieties. Within the larger bodies, where cooling may be considered to
have been slower, a series of rock types differing from one another in
mineral composition, and consequently in physical appearance, are
associated. At many localities these differentiated varieties are
intimately intermingled, so that within an area of a few square yards
almost the whole series may be found. Conditions of this sort were first
and best observed in the Lett properties on Wapus creek, where the
extensive stripping and trenching greatly facilitated geological study.
A suite of specimens was obtained which exhibit an unbroken gradation
from ordinary gabbro to the fine grained pink rock known as aplite. With
a decreasing pyroxene content and increasing abundance of feldspar the
rock grades from a dark green diabase at the basic end through a reddish
phase into a type which, in the field, might be termed a syenite, being
of granitic texture, red colour, and without visible quartz. These
phases are cut by aplite dikes which at first glance do not very closely
resemble them, but their comparison has shown that they too include a
group whose coarsest and most basic form does not differ greatly from
the syenitic type of the diabase series. From a pale flesh-coloured rock
of fine granitic texture in which a little dark mineral is visible, the
successive phases of the aplite graduate toward a dike material of light
pink colour and saccharine texture almost devoid of ferromagnesian
constituents.
_Microscopic character._—The consanguinity of diabase and aplite is
further established by microscopic study. It was intended to make a
somewhat complete comparative study; but, at the outset, the materials,
although fresh looking in the hand specimens, were found to be much
decomposed, sometimes so completely that the original composition could
not be satisfactorily inferred, and an outline must suffice therefore
until fresher material is secured.
The common gabbro type consists essentially of long prisms of
plagioclase embedded in light reddish augite. This ophitic structure,
upon which the distinction between diabase and gabbro depends, is well
developed, but does not show in hand specimens, so that for field use
the distinction is impracticable; ordinarily the term gabbro is applied
to the coarse grained, and diabase to the medium and finer grained
varieties. The plagioclase of the coarse grained specimens could not be
identified, being entirely altered to a coarse saussurite in which the
epidote was aggregated into large grains. The augite is almost equally
changed to strongly pleochroic hornblende possessing green and blue
green pleochroic tints. This hornblende is not a fibrous variety but
forms compact individuals, hence in the specimens studied it could not
be certain that some of it was not primary. In some cases it is further
altered to chlorite. Reaction between plagioclase and pyroxene seems to
have taken place, for chlorite occurs among the plagioclase
decomposition products as well as those of the pyroxene. Next to these,
ilmenite is the most abundant constituent, occurring as irregular masses
largely altered to leucoxene in which the original ‘gridiron’ structure
is distinct. Quartz is present in subordinate amounts usually in
micrographic intergrowth with the plagioclase. Small, well defined
hexagonal rods of apatite, titanite crystals, and rare zircons are also
present. The structure and mineral constitution render the term
quartz-diabase appropriate. Finer grained specimens secured from dikes
were found to be much fresher than the coarser types and yielded more
satisfactory thin sections; the ophitic structure is more pronounced,
but the mineral composition is the same. The plagioclase laths of one
section were determined optically to be an intermediate labradorite.
Small flakes of biotite partially altered to chlorite occur accessorily.