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Updated: June 5, 2025
These pseudo-fragments of shale will perhaps explain, in some cases, the origin of apparently extraneous fragments in crystalline metamorphic rocks. I mention this, because I found near Rio de Janeiro a well-defined angular fragment, seven yards long by two yards in breadth, of gneiss containing garnets and mica in layers, enclosed in the ordinary, stratified, porphyritic gneiss of the country.
These figures are of a far superior class of sculpture to the idols usually seen in Ceylon, especially that in the reclining posture, in which the impression of the head upon the pillow is so well executed that the massive pillow of gneiss rock actually appears yielding to the weight of the head.
On the 27th of July we embarked at four A.M., and proceeded along the south shore of the lake, through a narrow channel, formed by some islands, beyond the confluence of the principal branch of the Slave River; and as far as Stony Island, where we breakfasted. This island is merely a rock of gneiss, that rises forty or fifty feet above the lake, and is precipitous on the north side. As the day was fine, and the lake smooth, we ventured upon paddling across to the Rein-Deer Islands, which were distant about thirteen miles in a northern direction, instead of pursuing the usual track by keeping further along the south shore which inclines to the eastward from this point. These islands are numerous, and consist of granite, rising from one hundred to two hundred feet above the water. They are for the most part naked; but towards the centres of the larger ones, there is a little soil, and a few groves of pines. At seven in the evening we landed upon one of them, and encamped. On the following morning we ran before a strong breeze, and a heavy swell, for some hours, but at length were obliged to seek shelter on a large island adjoining to Isle
These facts appear to me opposed to the theory, that rock-salt is due to the sinking of water, charged with salt, in mediterranean spaces of the ocean. It is worthy of notice, that the foliation of the gneiss and mica-slate, where such rocks occur, certainly tend to run like the metalliferous veins, though often irregularly, in a direction a little westward of north.
In the same region the Carboniferous limestone attains so unusual a thickness namely, more than 4000 feet as to appear to compensate in some measure for the deficiency of contemporaneous sedimentary rock. Laurentian gneiss. 2. It is admitted that when two formations are unconformable their fossil remains almost always differ considerably.
In the very centre of the range, where the table-land attains its highest elevation, great masses of intrusive diorite and granite occur; and the latter is found in dykes piercing the gneiss and sub-metamorphic series throughout the southern half of the boundary of the plains.
At Bahia, in Brazil, we have seen that a true injected hornblendic dike, not only has suffered metamorphosis, but has been dislocated and even diffused in the surrounding gneiss, under the form of separate crystals and of fragments. I have described these islands in a paper published in the third volume of the "Geological Journal."
But if we investigate different mountain chains, we find gneiss, mica-schist, hornblende-schist, chlorite-schist, hypogene limestone, and other rocks, succeeding each other, and alternating with each other in every possible order.
The rock on which the boulder formation reposes, if it consists of granite, gneiss, marble, or other hard stone, capable of permanently retaining any superficial markings which may have been imprinted upon it, is usually smoothed or polished, like the erratics above described, and exhibits parallel striae and furrows having a determinate direction.
Granite and gneiss, which are of like chemical composition, have been shown, in various cases, to pass one into the other; as at Valorsine, near Mont Blanc, where the two, in contact, are observed to "both undergo a modification of mineral character.
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