W. H. Collins

“ 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 Archæan possessed a peneplanated surface, not greatly different from the present one, which is well preserved where overlain by erosion remnants of Huronian sedimentary rocks, but which at other points has been further denuded. The Lower Huronian rocks are of clastic nature, consisting in ascending order, of conglomerate, greywacke, slate and quartzite, which pass conformably into an upper conglomerate; while a granite-like, arkose member is believed from its similarity to rocks of the same character in the Cobalt area, to be possibly of later, Middle Huronian age. ”

“ 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. ”

“ 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.
The aplite is also so much weathered that doubt sometimes exists as to its original composition. ”

“ In general the veins seen on Duncan lake are exceptionally rich in quartz, and gash veins are common. Chalcopyrite, pyrite and galena are the most noticeable metalliferous minerals, but cobalt bloom stains are frequently observable. So far as known no attention has been given to the large diabase body between Duncan lake and the West branch, although its size and varied composition are thought to make it a desirable prospecting ground. ”

“ 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 ”

“ 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. ”

“ A few chains eastward the gneiss bands are wider, and, at a distance of 10 chains, hornblende granite is continuous.
Where terminated by diabase the contact is definite and not marked by notable alteration on either side. Contacts with the Huronian are equally sharp and unconformable, in every case the Laurentian disappearing beneath the sedimentary formation; at the south end of Kenisheong lake the Huronian has been trenched to a depth sufficient to expose the underlying gneissic floor near the water’s edge. ”

“ Pleistocene deposits are thin, and nearly everywhere the rock formations are well exposed.
A basement complex underlies the entire region, either appearing at the surface or hidden beneath areas of Huronian sediments. This basement consists largely of Laurentian biotite and hornblende gneisses, with patches of vertically foliated, Keewatin schists caught up in the former; the intervening contacts forming indefinite zones, in which intrusive action is manifested. In this report, for convenience, this complex will be referred to as the Archæan. ”

“ 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 34square yards almost the whole series may be found. ”

“ A prominent hill of the same kind is visible from Obushkong lake, lying a short distance to the north-west of that body. Just west of Mosher lake as represented in fig. 5, two round hills of about equal size rise 300 feet above the water level. The more southerly of the two is of Keewatin, while that to the north is composed wholly of diabase, Huronian lying around the base of each. Bold, but less individualized elevations are common in the neighbourhood of Kenisheong lake, and other localities. All these hills are markedly rocky and free from soil. ”

“ Leucoxene representing ilmenite is surprisingly abundant considering the scarcity of iron bearing silicates. Apatite is an abundant accessory mineral, forming small prisms, while zircon crystals are rare. The rock in the specimens examined is remarkably rich in calcite, which in amount ranks next to the plagioclase.
Comparison of sections from specimens intermediate between the aplite and the ordinary diabase brings out some features of the differentiating process. The governing changes are in the proportion of pyroxene, and in the composition of the plagioclase. ”

“ 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. ”

“ 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. ”

“ It is also reported that preliminary surveys for the extension of the railway to Gowganda are in progress. Meanwhile a business centre is springing up on the east side of Gowganda lake. A sawmill was put in operation on February 3, but has since been stopped owing to its location within a government timber reservation. A town plot has been laid out at the foot of the lake and lots are now purchasable from the Ontario Department of Lands, Forests and Mines. Buildings are being erected as rapidly as the supply of material permits. ”

“ 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. ”

“ Usually the contact edge of the diabase has been deeply eroded along with the adjoining rock to form narrow ravines, but wherever it persists a slight chilling is perceptible. The 38fine grained edges are much more apparent in dikes where the mass of hot material being much less was more susceptible to the influence of cold surroundings. The intrusive nature is also demonstrated by angular blocks of Huronian materials enclosed within the diabase, instances of which may be seen on Wapus creek. ”

“ 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. ”

“ 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 15resistant and uneven rock floor in which the streams are unable to carve channels for themselves. ”

“ Like the other formations of the region, the Laurentian is cut by diabase dikes of post-Huronian age which, owing to their dark colour, are conspicuous among the lighter granite rocks. In the neighbourhood of Zigzag lake they are abundant, and are usually from twenty 25to sixty feet or more in width, the smaller of which are not large enough to map. ”

“ Between Obushkong and Firth lakes and toward L’Africain lake it is largely in contact with Keewatin. This contact is a vaguely defined zone rather than a line, the formations being separated by the intermediate strip containing mingled portions of both. Proceeding across this strip from the Laurentian to the Keewatin, the gneisses of the former become charged with ribbons of highly crystalline schist, already described. Near the Keewatin edge these increase in quantity and sometimes appear as tongue-like protrusions of that system. This condition exists along the east of L’Africain lake ”

“ A poikilitic arrangement is more or less distinct; the feldspar is also micrographically intergrown with quartz. No orthoclase could be identified in any of the specimens. Quartz does not appear to be much more abundant than in the diabase. The only recognizable ferro-magnesian mineral is a strongly pleochroic reddish brown biotite, which occurs sparingly in small ragged flakes, partly altered to chlorite. ”

“ ASBESTOS. Distribution. In the Keewatin area between Firth and Obushkong lakes there occur masses of a basic igneous rock through whose decomposition serpentine and asbestos have been developed. The localities given in connexion with the description of the Keewatin may be briefly restated. Two bodies were found. One of these, lying east of Foot lake and 20 chains from Obushkong was traced for a width of 4 chains, but nothing learned of its north and south extent. It consists very largely of green serpentine traversed by a network of fine, white weathering veins of asbestos. ”

“ The distribution 9of the veins so far as known is confined to the larger diabase areas, the dikes and smaller bodies being undifferentiated and unmineralized; but the Huronian adjacent to the diabase also contains veins, somewhat more siliceous, yet evidently of the same age as the others. Alteration and impregnation of the country rock has taken place to an unknown, but, presumably, limited extent. Some of the veins are remarkably rich, and many of them occupy persistent, well defined fissures. ”

“ At the first rapid above Kenisheong lake the Huronian is cut by a broad dike of different character from those belonging to the quartz-diabase intrusion. It is a remarkably fresh compact rock of dark grey colour and medium texture in which an ophitic structure is prominent, acicular prisms of glassy feldspar penetrating the dark main mass. Its density is 2·991. Under the microscope it is holocrystalline and the constituent minerals occur in only one generation. The constituents as determined by linear measurements were found to be plagioclase, 67·5 per cent ”

“ Local description.—Diabase is most abundantly exposed in the area between Gowganda lake and the portage route connecting Firth and Elkhorn lakes, where it forms three parallel tongues of approximately equal dimensions. Coarse grained types prevail in which the differentiation of acid phases is pronounced. Aplite dikes up to 10 feet in width, fairly coarse grained and sometimes tolerably rich in dark constituents, are abundant. ”