DEFORMATION OF ROCKS 349 
obtain between its strike and that of bedding should be ascer- 
tained, and if discrepancies are found this indicates a complexly 
folded district. 
(6) Pumpelly also formulated the principle that “The degree 
and direction of the pitch of a fold are indicated by those of the 
axes of the minor plications on its sides.”” This statement must 
be understood to apply to the direction and pitch of the pri- 
mary fold at the point where the secondary fold is observed. 
This principle is a direct corollary from the relations of cross 
folds as given on a previous page, and it is of the greatest service 
in determining the structures of very complexly folded districts, 
because in them minor plications are so numerous. They may 
be seen in their entirety, and may therefore give the required 
determination of the character of the cross folds. The principle 
is, however, only approximately true. It would be wholly true 
if the secondary folds upon the flanks of the primary fold were 
exactly of the same character as the latter. But since the forces 
locally vary in direction and amount, and the rocks vary in rigid- 
ity, the direction and pitch of a secondary fold may vary some- 
what from those of a primary fold. Usually this deviation is so 
small that the principle is invaluable in field work in regions of 
complex folding, and gives data of sufficient accuracy for ordi- 
nary purposes. 
It is evident that in the application of all the above criteria 
we must consider bedding and not secondary structure. The 
criteria upon which this discrimination is made will be consid- 
ered in a later paper. 
Very often in regions of complex folding observers note only 
the most conspicuous folds in a single direction. The fact that 
folds are composite may be overlooked, and that they are com- 
plex is even less likely to be seen. The difficulty is further 
increased because of faults and secondary structures, such as 
slatiness, schistosity, and banding, which may be mistaken for 
5) 
bedding. The development of these structures and their rela- 
tions to folds and bedding will be considered in following papers. 
In ordinary districts where there are cross folds the more 
