THE ORIGIN OF MOUNTAINS. 
to form a kind of wall, sometimes so thin that it is 
actually pierced by a natural hole, as for instance 
the Martinsloch above Elm, in Glams. There is 
another of these orifices near the summit of the 
Pilatus, one in the Marchzahn, a mountain of the 
Gastlose chain, and another in the Piz Aela, also 
known for that reason as Piz Forate, between the 
Albula and the Oberhalbstein Rhine.* 
When we look at these abrupt folds and com- 
plicated contortions, the first impression is that they 
must have been produced before the rocks had 
solidified. This, however, is not so. They could not 
indeed have been formed except under pressure. 
We must remember that these rocks, though they 
are now at or near the surface, must have been 
formerly at a great depth, and where the pressure 
would be tremendous. Even in tunnels, which of 
course are comparatively near the surface, it is some- 
times found necessary to strengthen and support the 
walls which would otherwise be crushed in. The 
roadways in coal-mines are often forced up, especially 
where two passages meet. This indeed is so com- 
mon that it is known as the “creeps.” In deep 
tunnels it has not unfrequently happened that when 
strata have been uncovered they have suddenly bent 
* Theobald’s Graubiinden, Beitr. z. Geol. Karte d. Schw n. 
