418 ROLLIN T. CHAMBERLIN 
below, thus maintaining the general wedge shape. The deepest 
folds lie beneath the middle of the belt. 
In the broader study of diastrophism it is to be recognized that 
movement is not wholly confined to the strongly deformed masses, 
but takes place in some less degree in the masses that lie below 
them and at their sides. Associated movements are to be recog- 
nized which extend deep into the earth and possibly even through- 
out its whole mass. ‘The relations between the distinctly distorted 
section and the environing portion are various. For example, the 
bordering thrust-faults on the margins of the southern Appalachi- 
ans and of many other strongly compressed mountain ranges 
indicate that actual fracturing and shear take place very commonly 
between the strongly deformed and the slightly deformed blocks 
near the surface. Where the deformation has not been so intense, 
a sharp upturn of the strata in a great fold may mark the borders 
of the mountainous belt, as at Tyrone, Pennsylvania, and in the 
Colorado Rockies. Here actual fracture has not developed to any 
important extent, though there has been an approach toward 
fracture on the outer limbs of such folds. The adjustment between 
the more movable, more deformed portion and the less movable, 
less deformed region has been accomplished, partly by shearing 
and partly by mass rearrangements taking place in the folding 
process. With increasing depth below the surface, actual fault- 
ing should diminish, though distributive shear should presumably 
descend much deeper. No limiting depths can be assigned, for 
the time element plays an important part, though not easy to 
evaluate. To quick-acting stresses the earth reacts as an elastico- 
rigid body; under long-continued stresses it yields to slow mass 
movement. With greater depth molecular rearrangement and 
recrystallization should presumably take precedence. ‘These might 
be manifested by folding, or by cleavage, or perhaps-only by rock 
flow. Under such conditions the deformed block would probably 
not be sharply bordered. 
While the border belts near the surface are in places actual 
fault planes, nevertheless, throughout most of their extent they 
probably constitute zones, perhaps of considerable breadth, sepa- 
rating the more deformed mountain mass from the less deformed 
