Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/458

448 Ketterin, the nature of the rock alters, and it assumes an aspect approaching here and there to mica slate, but it is still characterized in most places by the true graywacke structure, that is to say, by grains united by a cement, the cement in this case consisting of micaceous schist instead of clay slate. At Ben Ledi the structure still more resembles that of mica slate, insomuch that but for the previous examination of the adjoining district, we should not suspect it to consist of any thing but genuine mica slate. Beyond this, and to the east of Loch Lubnaig, occurs true mica slate which constitutes the whole of this district as far as Perth. I may remark too, that the graywacke of which the character is so perfect in the lower parts of the vale of Loch Ard, becomes micaceous as we ascend the course of the stream, and that in the upper parts of the valley, it is often very difficult to ascertain under which head it is to be ranked.

Those who have been engaged in similar observations on an extensive tract of country, well know how difficult it is to fill up all the chasms which the nature of the ground forms in the pursuit of a series of rocks. Yet I have very little doubt that a true gradation exists here, between mica slate and graywacke slate, and that the usual intermediate member, primitive clay slate, does not occur as a necessary part of the series, but that it is irregularly alternated with the graywacke. It was with a view to these rocks, that I suggested the propriety of extending the definition of graywacke, so as to include those which contain mica slate, talc slate, and chlorite slate, as the cement of the grains, instead of limiting it to clay slate alone. In many of the rocks which occur here, the cementing ingredient appears to consist of these several substances, the rock in