THE ORIGIN OF MOUNTAINS. 
75 
therefore infer, from the absence of a given stratum 
in such cases, that it never existed. 
In many cases the rock is broken up into flat or 
more or less lenticular pieces, which have been 
squeezed over one another so that their surfaces 
have been rendered smooth and glistening. Such 
surfaces are known as slickensides. This process has 
sometimes been so intense and so general that hardly 
a piece can be found which does not present such a 
polished surface. The particles of stone which now 
touch were once far apart, others which are now at 
a distance once lay close together. The cracks, move- 
ments, and friction which result in such a structure 
must from time to time produce sounds, and the 
mysterious subterranean noises sometimes heard are 
perhaps thus produced. 
Fig. 1 6 represents a section of Rothidolomite, 
and it will be observed that, as we should expect 
theoretically (see also Fig. 12 , p. 68), the strata are 
thinnest in the limbs, where they are squeezed out, 
and broader in the arches. This is visible in great 
mountain folds, as well as in hand specimens. 
In the part of the curve where the effect of the 
force is to draw out the strata, they will as shown 
above, if capable of giving way, become thinner. If 
however they are not plastic they must crack, the 
