. Elements of geology. Geology. Fig. 71. Strata of chert, grit, and marl, near St. Jean de Luz. used for ridge-tiles on the roof of a house. Although this chert is now brittle, we must necessarily suppose that it was flexible when folded into this shape; nevertheless it must have had some solidity, for precisely at the angle of flexure there are numerous cracks filled with calcedony. There are also some veins of quartz, b. Fig. 70., traversing the same formation which have filled irregular fissures, probably enfiltered at the same time as the calcedony above mentioned. Between San Caterina and
. Elements of geology. Geology. Fig. 71. Strata of chert, grit, and marl, near St. Jean de Luz. used for ridge-tiles on the roof of a house. Although this chert is now brittle, we must necessarily suppose that it was flexible when folded into this shape; nevertheless it must have had some solidity, for precisely at the angle of flexure there are numerous cracks filled with calcedony. There are also some veins of quartz, b. Fig. 70., traversing the same formation which have filled irregular fissures, probably enfiltered at the same time as the calcedony above mentioned. Between San Caterina and Castrogiovanni, in Sicily, bent and undulating gypseous marls occur, with here and there thin beds of solid gypsum interstratified. Sometimes these solid layers have been broken into detached fragments, still preserving their sharp edges, (g Fig. 71.), while the continuity of the more pliable and ductile marls, m has not been inter- rupted. I shall conclude my remarks on bent strata by stating, that, in mountainous regions like the Alps, it is often difficult for an experienced geologist to deter- mine correctly the relative age of beds by superposition, so often have the strata been folded back upon themselves, the upper parts of the curve having been removed by denudation. Thus, if we met with the strata seen in the section Fig. 72., we should naturally suppose that there were twelve distinct beds, or sets of beds. No. 1. being the youngest, and No. 12. the oldest of the series. But this section may, perhaps, exhibit mere- ly six beds, which have been fold- ed in the manner seen in Fig. 73., so that each of them are twice repeated, the position of one half being reversed, and part of No. 1., originally the uppermost, having now become the lowest of the series. These phenomena are often observable on a magnificent scale in certain regions in Switzerland, where there are precipices from 2000 to 3000 feet in perpendicular height. In the Iselten Alp, in the valley of the. g. gyp
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