Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/169



The most leading error is that which states the promontory of Sleat as composed of micaceous schistus. This substance occurs in several places, and often in distinct beds of considerable thickness, while in others it consists of mere laminæ interposed among the other rocks. These rocks are of very various composition, but as gneiss occurs in considerable quantity among them, exceeding greatly the space occupied by the micaceous schist, the latter will probably be considered by most geologists as subordinate to the former, and the gneiss as being the lowest and fundamental rock of Sky. But the truth is, that this series is not amenable to any systematic rules, and it will be better to state the fact as it exists, than to incur the risk of a similar error by transferring to gneiss that rank which I had before conferred on micaceous schist. It will be seen that the series presents anomalies which cannot be removed by any theory of subordination, and that the history of the district of Sleat is not the least interesting part of the unexpected appearances presented by this very instructive island.

The characters of the gneiss are so strongly marked in one part of the space which it occupies, that there can be no difference of opinion respecting it. Besides mica or hornblende it contains a conspicuous proportion of high red felspar and quartz, being at the same time distinctly laminated, and from the contrast of its colours, very remarkable.

From this regularity of structure and composition it passes into a substance for which there is no name in the present nomenclature of rocks, and which can only be ranked with gneiss by assuming a considerable latitude of character. This rock is a compound of felspar and quartz with chlorite schist, these substances being still interlaminated as before, so that each mineral generally occupies a distinct portion of the specimen, the latter becoming substituted