MOUNTAINS AND THEIR ORIGIN. 101 
since it has broken its way through them, and 
forced them out of their natural position ; and it 
must have been previous to the Jurassic and Cre- 
taceous deposits, since they have been accumu- 
lated peacefully at its base, and have undergone 
no such perturbations. 
The task of the geologist would be an easy 
one, if all the problems he has to deal with were 
as simple as the case I have presented here ; but 
the most cursory glance at the intricacies of 
mountain-structure will show us how difficult it 
is to ti ace the connection between the phenom- 
ena. W e must not form an idea of ancient 
mountain-upheavals from existing active volca- 
noes, although the causes which produced them 
were, in a somewhat modified sense, the same. 
Our present volcanic mountains are only chim- 
neys, or narrow tunnels, as it were, pierced in 
the thickness of the earth’s surface, through 
which the molten lava pours out, flowing over 
the edges and down the sides and hardening 
upon the slopes, so as to form conical elevations. 
The mountain-ranges upheaved by ancient erup- 
tions, on the contrary, are folds of the earth’s 
surface, produced by the cooling of a compara- 
tively thin crust upon a hot mass. The first 
effect of this cooling process would be to cause 
contractions ; the next, to produce corresponding- 
protrusions, — for, wherever such a shrinking and 
