The aplite is also so much weathered that doubt sometimes exists as to its original composition. Thin sections consist very largely of plagioclase and quartz, coloured constituents being small in quantity. The plagioclase is twinned according to albite, pericline, and carlsbad laws, and in one case a baveno twin was observed; optically it behaves as almost pure albite and is decomposed to sericite instead of epidote. 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. 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. A specimen somewhat paler in colour than the typical diabase was found to contain considerably less augite and more abundant plagioclase, the other constituents remaining fairly constant. The plagioclase was an acid andesine of the composition (Ab 65 An 35). A still lighter coloured type possessed oligoclase (Ab 72 An 28) and a small amount of blue green hornblende, representing the wholly altered pyroxene. Orthoclase could not be found in any of the sections, so that none of these rocks can be properly called syenite or granite.
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. The rugged surface of this part of the country is largely due to the presence of the diabase, the peculiar arrangement of which has produced a constant system of north and south ridges. The contacts with the Huronian occupy the bottoms of gullies, with the main body of diabase forming high ground. This highly relieved and consequently well exposed surface has greatly facilitated the exploitation of this area, as indeed is the case for the whole district. Terminally each of the three bodies tapers out or forks, but these details were not closely mapped. In the case of the most easterly one the portions reaching the east side of Firth lake are darker and more decomposed than the ordinary diabase, and during the field operations were considered of other character. Laboratory study of the specimens shows them, however, to be basic diabase in which hornblende has completely replaced the pyroxene. In consequence of this misconception the connexions of the exposures on Firth lake were not well worked out, but they almost certainly lie as shown in the map and may form a continuous connexion with the large diabase body farther north. The two areas north-east of Firth lake present much the same appearance as those of the Gowganda district.
The most extensive body is that which coincides in direction with Duncan lake, resembling a great hook, the shank of which is traceable for twelve miles, the crooked end lying about midway between the West branch and Firth lake. Neither termination was found, on account of the swampy or sandy character of the country, so that the present representation may not be complete. Just east of L’Africain lake there are some diabases which may be continuations of it, but owing to their altered appearance they could not be distinguished with certainty in the field from similar Keewatin forms. Along Duncan lake it appears to represent the edge of a sill about 300 feet thick lying in the bedding plane of the Huronian sediments which lie both above and beneath it. From the attitude of the latter the sill is believed to dip eastward at an angle of 30 or 40 degrees. The exposed width is usually less than in the Gowganda area and its composition is more homogeneous, but at the wider portions the same intermingling of basic and acid phases and aplite dikes obtains, as for example where it crosses Wapus creek. The full extent of that portion which extends north-westward from Mosher lake is not known. The formation at that point is obscured by gravelly soil; it appears, however, to be unusually wide and well differentiated. An apparently large body of diabase lies between the north part of Duncan lake and the West branch. The mapping of this was left incomplete, so that its total southern extent cannot be given. Its northern part presents nothing unusual, but about Vipond lake a syenitic phase is developed, probably indicating a mingling of differentiated rock varieties as at Gowganda lake or Wapus creek.
The remaining diabase bodies are dikes. Probably these are very numerous and widespread, but this can only be determined by work of much more widespread nature than the economic possibilities of such an investigation warrant at present. They are most frequently seen in the Laurentian, probably not so much because they are more numerous there, but because they are more conspicuous in the granites than in the Keewatin schists and Huronian, from which they differ less in colour. They attain widths of 60, 100 or more feet, in which cases the texture is like that of the large masses, but differentiated types and aplite dikes do not seem to accompany them. The smaller dikes, diminishing to a width of 1 foot, are compact black rocks of microcrystalline habit, but like the gabbroid varieties mineralogically.
Age.—The time of intrusion of these rocks can only be defined as post-Huronian or post Middle Huronian, there being no younger formations with which to correlate them. They are identical with the diabase in other parts of the Montreal River region and of Cobalt. As already noted the larger masses have produced very limited metamorphic changes in the adjoining rocks, developing incipient adinole zones in the Huronian greywackes, besides hardening and fracturing them. 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 fine 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.
Olivine Diabase.
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; olivine, 14 per cent; augite, 1·5 per cent; iron ore, probably ilmenite, 5 per cent; apatite, 1·8 per cent; biotite, 1·3 per cent; and zircon, 0·03 per cent. The rock is remarkably fresh, even the olivine showing no signs of decomposition. The plagioclase was determined by optical methods to be a labradorite of the composition Ab 1 An 1. It forms laths, twinned according to albite, pericline and carlsbad laws, which penetrate the ferromagnesian minerals. A few large crystals show fine zonary lamellation. Olivine is in idiomorphic or rounded grains, occasionally bordered by a little rim of biotite, possibly a product of reaction during the period of crystallization. The augite is reddish brown in colour and later than either olivine or plagioclase, filling the interstices between the feldspar laths. Irregular masses of black iron ore, probably ilmenite, are scattered throughout the section, in some cases showing good crystal forms. A deep brown strongly pleochroic biotite with a very small optical angle occurs accessorily in small shreds. Apatite is mostly in slender but sometimes stout hexagonal prisms. Minute crystals of zircon are rare.
The rock may be designated an olivine diabase of markedly fresh aspect. It penetrates the Huronian, but judging by its unusually fresh condition it must be comparatively young.