86 
ELEMENTS OF GEOLOGY. 
At *the western extremity of the Pyrenees, great curva¬ 
tures of the strata are seen in the sea-cliffs, where the rocks 
consist of marl, grit, and chert. At certain points, as at a. 
Fig. 70, some of the bendings of the flinty chert are so sharp 
Fig. 70. 
Strata of chert, grit, aud marl, near St. Jean de Luz. 
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 great¬ 
est flexure, small cracks, which show that it was solid, and 
not wholly incapable of breaking at the period of its dis¬ 
placement. The numerous rents alluded to are not empty, 
but filled with chalcedony and quartz. 
Between San Caterina and Castrogiovanni, in Sicily, bent 
and undulating gypseous marls occur, with here and there 
thin beds of solid gypsum inter- 
stratified. Sometimes these solid 
layers have been broken into de¬ 
tached fragments, still preserving 
^ their sharp edges (^, Fig. 71), 
^ while the continuity of the more 
pliable and ductile marls, m, m, ' 
9 has not been interrupted. 
We have already explained. Fig. 
69, that stratified rocks have usu¬ 
ally their strata bent into parallel folds forming anticlinal 
and synclinal axes, a group of several of these folds hav¬ 
ing often been subjected to a common movement, and having 
acquired a uniform strike or direction. In some disturbed 
Fig. 72. 
regions these folds have been doubled back upon themselves 
in such a manner that it is often difficult for an experienced 
geologist to determine correctly the relative age of the beds 
by superposition. Thus, if we 
meet with the strata seen in the 
section. Fig. 72, we should nat¬ 
urally 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 
