THE MOUNTAINS OF SWITZERLAND. 
97 
spent, and that slow movements may be still in pro- 
gress along the flanks of the mountains.* 
Many of these earthquakes are very local and as 
a rule not deep seated, at a depth of not more than 
from 1 5,000 to 20,000 metres. 
Even however in the Central Alps there is some 
evidence of present strain. When the tunnels were 
being pierced for the St. Gotthard line, and especi- 
ally the Wattinger tunnel near Wasen, slight ex- 
plosions were often heard, and blocks of rock were 
thrown down on the workmen. These generally came 
from the roof, but sometimes from the sides, and 
eventually it was found necessary to case the interior 
of the tunnel.** These phenomena, however, may 
have been only due to the great pressure. 
The American geologists, and especially Dana, 
have pointed out that folded mountains are not as a 
rule symmetrical but one-sided. Suess*** has ex- 
tended this to Switzerland, and indeed to folded 
mountains generally. It is remarkable that in all the 
European mountain systems — the Alps, Appennines, 
Jura, Carpathians, Hungarian Mountains, etc., the 
outer side of the curve presents a succession of folds 
* Heim, Mech. d. Gebirgsb., vol. n. 
** Baltzer, Beitr. z. Geol . B. d. Schw., L. XXIV. 
*** Das Antlitz der Erde. 
Scenery of Switzerland. I. 
7 
