72 
SCENERY OF SWITZERLAND. 
and cracked, which shows that they were under 
great lateral pressure. Yet the deepest mine only 
reaches 800 metres. 
Treska* has shown by direct experiment that 
the most solid bodies, lead, tin, silver, copper, and 
even steel, will give way and “flow” under a pressure 
of 50,000 kilograms per square centimetre. More- 
over, there is direct and conclusive evidence that the 
Swiss rocks were folded after solidification. Tn many 
cases contorted rocks contain veins (Fig. 16) which 
are in fact cracks filled up with calcite, etc. Such 
fine fissures, however, can only occur in hard rock. 
Again the Eocene contains rolled pebbles ot Gneiss, 
Lias, Jurassic, etc., which must therefore have be- 
come hard and firm before the Eocene period,** 
while the folding did not occur till afterwards. It is 
clear therefore that when the folding took place the 
rocks were already solidified. No doubt, however, 
the folding was a very slow process. It took place, 
and could only take place, deep down, far below the 
surface, under enormous pressure, and where the 
material was perhaps rendered somewhat more plastic 
by heat. In the later and higher rocks we find com- 
pression with fracture, in the earlier and lower rocks 
* Comptes Rendtis, 1874. 
** Heim, Mech. d. Gebirgsb., vol. n. 
