Note 23 — Chapter VI · PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION.

I observed in some parts that the tops of the laminæ of the clay-slate (b of the diagram) under the superficial detritus and soil (a) were bent, sometimes without being broken, as represented in the accompanying diagram, which is copied from one given by Sir H. De la Beche (p. 42 “Geological Manual”) of an exactly similar phenomenon in Devonshire. Mr. R. A. C. Austen, also, in his excellent paper on S.E. Devon (“Geolog. Transact.,” vol. vi, p. 437), has described this phenomenon; he attributes it to the action of frosts, but at the same time doubts whether the frosts of the present day penetrate to a sufficient depth. As it is known that earthquakes particularly affect the surface of the ground, it occurred to me that this appearance might perhaps be due, at least at Concepcion, to their frequent occurrence; the superficial layers of detritus being either jerked in one direction, or, where the surface was inclined, pushed a little downwards during each strong vibration. In North Wales I have seen a somewhat analogous but less regular appearance, though on a greater scale (London Phil. Mag., vol. xxi, p. 184), and produced by a quite different cause, namely, by the stranding of great icebergs; this latter appearance has also been observed in N. America.