Note 47 — Chapter IX

"Some idea may be formed of the nature of these incurvations by supposing the gneiss beds to have been in a plastic state, either from the action of heat or of some other unknown cause, and, while in this state, to have been subjected to pressure at the two extremities, or in some other parts, according to the nature of the curvatures. But even this hypothesis (though the best that has been thought of) will scarcely enable us to explain all the contortions which not merely the beds of gneiss, but likewise of mica slate and clay slate, and even greywacke slate, exhibit. There is a bed of clay slate near the ferry to Kerrera, a few miles south of Oban, in Argyleshire. This bed has been partly wasted away by the sea, and its structure exposed to view. It contains a central cylindrical nucleus of unknown length (but certainly considerable), round which six beds of clay slate are wrapt, the one within the other, so as to form six concentric cylinders. Now, however plastic the clay slate may have been, there is no kind of pressure which will account for this structure; the central cylinder would have required to have been rolled six times in succession (allowing an interval for solidification between each) in the plastic clay slate."—Outlines of Mineralogy, Geology, &c., by Thomas Thomson, M.D.