PART I. CHAPTER V. 



73 



Bent Strata in Pyrenees Reversed dip of Strata. 



Fig. 70. 



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. gypsum, m. marl. 



Fig. 72. 



