Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/446

From Wikisource
Jump to navigation Jump to search
This page has been validated.
Dr. Mac Culloch on the Geology of

The bed of limestone has been quarried near the eastern end of the lake, and if the reports of the quarry-men are to be trusted, extends in a direct line for many miles. This part of its history merits investigation, as the limestones which lie in the highland schistus, are generally of very limited extent. The stone itself is remarkable for containing hornblende. The sides of the bed are in contact with hornblende slate, or rather with that unnamed and common rock, consisting of a slaty mixture of quartz, mica, and hornblende.

Where the limestone comes in contact with the schist, the hornblende crystals penetrate it in such quantity as to blacken the compound. Towards the centre of the mass they diminish in number, and at length disappear.

Mica also accompanies the hornblende to a certain extent within the bed, and the limestone being itself of a pure white, a marble is thus produced of an ornamental nature, and bearing a considerable resemblance, as well as analogy, to the celebrated marble of Tirey. A substance very much resembling massive garnet may be perceived here and there united with it, but in so small a quantity as to render the determination of its true nature difficult. It is worthy of notice, that the limestone is frequently of a large platy fracture where in contact with the hornblende schistus, and that, these plates are not straight but flexuous.

Craig Cailleach near Killin.

This summit forms a part of the ridge separating the vales of the Tay, and Lyon, and is the next highest in elevation to Ben Lawers, the most conspicuous part of that ridge. The predominant rock of this ridge, is a very well characterized chlorite slate, which