DEFORMATION OF ROCKS 351 
with the forms and distribution of folds in all of the many differ- 
ent districts to which it has been applied, and at the same time 
accords with the principles of mechanics, appears to me to give 
to ita considerable degree of probability. Even if the explana- 
tion be not accepted, the forms of folds and the principles appli- 
cable to their study remain the same. Thus we havea classifica- 
tion of folds and an outline of methods for their study which will 
assist in determining the structure of the complexly folded dis- 
tricts and in preparing areal maps of them. 
CHANGES ACCOMPANYING FOLDING. 
Contemporaneous with rock folding, and in a large measure 
dependent upon it, other changes occur in rocks. As has been 
seen, crevicing and brecciation largely depend upon the same 
forces as does folding.. During the process of folding old min- 
erals are transformed into new ones. New mineral material 
enters from the outside. The minerals are rearranged and 
mechanically modified. Secondary structures, such as cleavage 
fissility, joints, and faults may develop. In short, during the 
folding process the rocks are to a greater or less degree meta- 
morphosed. 
RELATIONS OF FOLDS AND UNCONFQRMITY. 
The folding of a set of inferior formations in a more compli 
cated manner than that of another set of superior formations may 
indicate a structural break between the two, and consequently 
that the two sets of formations belong to different series. In 
order that this criterion may be applied, it must be conclusively 
shown that the supposed upper formations are really above the 
others. It must not be assumed that a formation at one side of an 
axis of plication is in a superior position because less folded, for 
in many regions close folds die out within a comparatively short 
distance in a direction transverse to them. This is the case 
along the Green Mountains, where the closely folded Lower Pale- 
ozoic rocks pass quickly, to the westward, to unfolded or very 
gently folded ones. The change here takes place so rapidly that 
it has been supposed by many geologists that the more closely 
