58 



EEVERSED DIP OF STRATA. 



[On. V. 



fluid matter which they contained in their minute pores, as before 

 described (p. 35), and in part to the permeation of sea- water while they 

 were yet submerged. 



At the western extremity of the Pyrenees, great curvatures of the 

 strata are seen in the sea cliffs, where th*e rocks consist of marl, grit, and 

 chert. At certain points, as at a, fig. 78, some of the bendings of the 



Fig. 78. 



Strata of chert, grit, and marl, near St. Jean de Luz. 



Fig. 79. 



flinty chert are so sharp, that specimens might be broken off, well fitted 

 to serve as ridge-tiles on the roof of a house. Although this chert 

 could not have been brittle as now, when first folded into this shape, it 

 presents, nevertheless, here and there at the points of greatest flexure 

 small cracks, which show that it was solid, and not wholly incapable of 

 breaking at the period of its displacement. The numerous rents alluded 

 to are not empty, but filled with chalcedony and quartz. 



Between San Caterina and Castrogiovanni, in Sicily, bent and undu- 

 lating 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 g, fig. 79), while the continuity of the 

 more pliable and ductile marls, m rn, has 

 not been interrupted. 



I shall conclude my remarks on bent 

 strata by stating, that, in mountainous re- 

 g. gypsum, m, marl. gj ons jj^e the Alps, it is often difficult for 

 an experienced geologist to determine correctly the relative age of beds 

 by superposition, so often have the strata been folded back upon them- 

 selves, the upper parts of the curve having been removed by denudation. 

 Thus, if we met with the strata seen in the section fig. 80, we should 



naturally suppose that there were twelve 

 distinct beds, or sets of beds, No. 1 being 

 the newest, and No. 12 the oldest of the 

 series. But this section may, perhaps, 

 exhibit merely six beds, which have been 

 folded in the manner seen in fig. 81, so that each of them is twice re- 

 peated, the position of one-half being reversed, and part of No. 1, origi- 

 nally the uppermost, having now become the lowest of the series. These 

 phenomena are often observable on a magnificent scale in certain regions 

 in Switzerland in precipices from 2000 to 3000 feet in perpendicular 

 height. In the Iselten Alp, in the valley of the Lutschine, between 



Fig. so. 



