GREATER PROBLEMS OF PHYSICAL GEOLOGY. t 61 
(1.) It is a remarkable fact that they occur among sedi¬ 
mentary beds of great and variable thickness, which were 
rather rapidly accumulated. They seldom, and, so far as I 
now recall, never occur among strata which are of small 
thickness, slowly accumulated with uniformity over large 
areas; and the theory requires that they should occur in 
the heavy deposits or along their margins, and should have 
their greatest development there, for the forces called into 
play must be proportional to the masses involved. 
(2.) They occur in their systematic form along the ancient 
shore lines. This is but another way of stating the preceding 
proposition. It has its uses, however, for in so far as the con¬ 
tinents have preserved approximately their old shore lines 
since the ages in which the plications were formed there is a 
conspicuous parallelism of the axes of plication to the neigh¬ 
boring coast. This is true of the Pacific coast of the United 
States. As regards the Appalachian plications, we have the 
remarkable fact that in paleozoic time the ocean lay to the 
west of those vast bodies of folded strata instead of to the 
east of them, as now. We must look to a paleozoic Atlantis 
for the origin of a great portion of those sediments. The 
flow of the earth was from west northwest to east southeast. 
(3.) The parallelism of the folds and their occurrence in 
long, narrow belts formed by horizontal forces acting in one 
direction become a consequence so obvious as to need no 
comment. It is in strong contrast with the contractional 
theory, which gives a force without any determinate direc¬ 
tion. 
(4.) Another important fact is that these systematic flexures 
were mainly formed at the times the sediments were depos¬ 
ited. This is a fact of geologic observation. The contrac¬ 
tional hypothesis gives no determinate time for the formation 
of these flexures. It holds up to us a process continuous 
through all geological time, proceeding at a rate which 
diminishes but slowly as the ages roll by. These plications, 
according to the isostatic theory, are the results of the dis¬ 
turbance of isostasy, and follow immediately upon that 
