MOUNTAINS 823 
plexity; for the facts are difficult of acquirement, and 
even when all available information is obtained, there 
still remains the necessity of assuming changes whose 
nature is not fully understood. So the physicists and 
geologists who have attacked this problem, have arrived 
at widely different results. In stating our present 
knowledge, it will serve us to pass in review, first of 
all, the phenomena to be explained. 
There is first the fact that mountains rise in ridges, 
which in the majority of the larger ranges have a nearly 
north and south direction. Along or near these lines 
there have been uplifts, often repeated again and again, 
and these appear to have been slow and frequently 
intermittent. 
For a long time before some of the ranges began to 
rise, there was a preliminary settling of the sea bottom, 
during which thick deposits of sediment were accumu- 
lated. In these cases, which are numerous, mountains 
have been made out of thick beds of sediment, long hid- 
den under water. These sediments have been mostly, if 
not entirely, deposited near the shore line, so that moun- 
tains are often formed from thick beds of shore-line sedi- 
ment which have gathered during a long-continued 
subsidence. In the Appalachians such conditions were 
present, these mountains being made out of beds whose 
thickness in some places became as great as 30,000 or 
40,000 feet ; and the same is true in many other chains. 
