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Updated: June 5, 2025
The next outcrop observed was on the portage from the Nascaupee River. The rock, a biotite granite gneiss having a strike N. 82 degrees E. is much weathered and split by the action of the frost, and marked by pockets of quartz, usually four or five inches in width. Between this point and Lake Nipishish the underlying rock differs only in being more extremely crushed and foliated.
There is some fine-grained syenitic granite, orbicularly marked by ferruginous lines, and weathering into vertical, cylindrical holes, almost touching each other. In the gneiss, concretions of granular feldspar and others of garnets with mica occur.
MM. Huc and Gabet allude to the same tradition under another form. There is nothing remarkable in the geology of Choongtam: the base of the hill consists of the clay and mica slates overlain by gneiss, generally dipping to the eastward; in the latter are granite veins, containing fine tourmalines.
We stopped at the mouth of the Rio Tomo. The Indians went on shore, to prepare their food, and take some repose. When we reached the foot of the raudal, it was near five in the afternoon. It was extremely difficult to go up the current against a mass of water, precipitated from a bank of gneiss several feet high.
Here and there peaks of porphyry or basalt pierced through the white winding-sheet like the bones of a skeleton; and at intervals fragments of quartz or gneiss, loosened by the action of the air, fell down with a faint, dull sound, which in a denser atmosphere would have been almost imperceptible. However, in spite of their courage, the strength of the little band was giving way.
On the conical summit are hard quartzoze porphyries, which have apparently forced up the gneiss and slates, which dip in all directions from the top, and are full of injected veins of quartz. Below 7000 feet, mica-schist prevails, always inclined at a very high angle; and I found jasper near Namtchi, with other indications of Plutonic action.
These spheres are disposed in layers parallel to the foliation of the gneiss: and are the result of a metamorphic action of great intensity, effecting a complete rearrangement and crystallization of the quartz and mica in parallel planes, whilst the felspar is aggregated in spheres; just as in the rearrangement of the mineral constituents of mica-schists, the alumina is crystallized in the garnets, and in the clay-slates the iron into pyrites.
These last-named rocks are auriferous in the Quebrada del Oro, near Guigue; and between Villa de Cura and the Morros de San Juan, in the mountain of Chacao. The gold is contained in pyrites, which are found sometimes disseminated almost imperceptibly in the whole mass of the gneiss,* and sometimes united in small veins of quartz.
In one little rocky dell the water gushed through a hole in a soft stratum in the gneiss; a trifling circumstance which was not lost upon the crafty Brahmins, who had cut a series of regular holes for the water, ornamented the rocks with red paint, and a row of little iron tridents of Siva, and dedicated the whole to Mahadeo.
These red deposits may be accounted for by the decomposition of gneiss and mica schist, which in the eastern Grampians of Scotland has produced a mass of detritus of precisely the same colour as the Old Red Sandstone.
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