CURVED STRATA. >77 
We may still more easily illustrate the elfects which a lat¬ 
eral thrust might produce on flexible strata, by placing sev¬ 
eral pieces of difterently colored cloths upon a table, and 
when they are spread out horizontally, cover them with a 
book. Then apply other books to each end, and force them 
Fig. 58. 
towards each other. The folding of the cloths (see Fig. 58) 
will imitate those of the bent strata; the incumbent book 
being slightly lifted up, and no longer touching the two vol¬ 
umes on which it rested before, because it is supported by 
the tops of the anticlinal ridges formed by the curved cloths. 
In like manner there can be no doubt that the squeezed stra¬ 
ta, although laterally condensed and more closely packed, 
are yet elongated and made to rise upward, in a direction 
perpendicular to the pressure. 
Whether the analogous flexures in stratified rocks have 
really been due to similar sideway movements is a question 
which we can not decide by reference to our own observa¬ 
tion. Our inability to explain the nature of the process is, 
perhaps, not simply owing to the inaccessibility of the sub¬ 
terranean regions where the mechanical force is exerted, but 
to the extreme slowness of the movement. The changes 
may sometimes be due to variation in the temperature of 
mountain masses of rock causing them, while still solid, to 
expand or contract; or melting them, and then again cool¬ 
ing them and allowing them to crystallize. If such be the 
case, we have scarcely more reason to expect to witness the 
operation of the process within the limited periods of our 
scientific observation than to see the swelling of the roots 
of a tree, by which, in the course of years, a wall of solid 
masonry may be lifted up, rent or thrown down. In both 
instances the force may be irresistible, but though adequate, 
it need not be visible by us, provided the time required for 
its development be very great. The lateral pressure arising 
from the unequal expansion of rocks by heat may cause one 
mass lying in the same horizontal plane gradually to occupy 
