130 
THE GROWTH OF CONTINENTS. 
caused by mountain-upheavals. The former are 
much more numerous than the latter, and in 
every epoch geologists have distinguished a num- 
ber of such changes in the surface of the earth, 
accompanied by the introduction of a new set of 
animals, though the changes in the organic world 
are not so striking as those which coincide with 
the mountain-upheavals. Still, to the eye of the 
geologist they are quite as distinct, though less 
evident to the ordinary observer. To these divis- 
ions it seems to me that the name of Period is 
rightly applied, because thoy seem to have been 
brought about by the steady action of time, and 
by gradual changes, rather than by any sudden 
or violent convulsion. 
It was my good fortune to be in some degree 
connected with the investigations respecting the 
limitation of Periods, for which the geology of 
Switzerland aliorded peculiar facilities. My early 
home was near the foot of the Jura, where I con- 
stantly faced its rounded domes and the slope by 
which they gently descend to the plain of Swit- 
zerland. I have heard it said that there is some- 
thing monotonous in the continuous undulations 
of this range, so different from the opposite one 
of the Alps. Put I think it is only by contrast 
that it seems wanting in vigor and picturesque- 
ness ; and those who live in its neighborhood be- 
come very much attached to the more peaceful 
