2 6o 
SCENERY OF SWITZERLAND. 
weather; but, on the other hand, the separate 
grains of sand or quartz are not only insoluble, but 
offer great resistance to mechanical action. Water, 
especially if charged with carbonic acid, can dissolve 
some Silica, but the quantity is insignificant. 
Calcareous rocks are much more readily at- 
tacked. They often contain some alumina and 
siliceous nodules, which remain as a reddish clay 
with flints after the calcareous matter has been re- 
moved. 
Argillaceous rocks cannot be dissolved, but they 
are in many cases readily reduced to fine particles 
and then easily removed. They generally contain 
some calcareous material, and when this is washed 
away, pores and hollows are left which let in 
moisture. Even when compressed into slates they 
often yield to the influence of moisture, and if suf- 
ficiently saturated sink into the form of mud. 
Along the sides of valleys calcareous rocks often 
present steep, even vertical, faces (see ante, Fig. 44, 
p. 184, Valley of Bienne). Sandstones and Granite are 
generally less bold, and marly beds assume still more 
gentle slopes. The behaviour of argillaceous beds is 
more dependent on circumstances; if they are fairly 
dry they bear themselves well, but if they become 
wet they are very perishable. 
