350 STUDIES FOR STUDENTS 
conspicuous set is generally chosen as giving the direction of 
folds, while the less conspicuous set in the other direction is con- 
sidered as giving the pitch of the folds. It does not follow that 
the folds giving the pitch, in the magnitudes of their vertical 
components, are less important than the more conspicuous lon- 
gitudinal folds, for the lowness of the dips of the transverse folds 
may be more than compensated by their greater lengths, and the 
cross folds may be of the first order of magnitude in a district. 
Usually it is possible to work out the structure of such a district 
without particular attention being directed to transverse folds. 
They are so gentle that the changes of strike and dip are not 
rapid, and a satisfactory map may be made without recognition 
of the existence of cross folds. It is suspected that the largest 
folds of a district have often escaped the attention of the geolo- 
gists who did the mapping. 
The more complex the folding of a district the more neces- 
sary it is in determining its structure to consider the character of 
both sets of folds, and for very complex districts this is impera- 
einer 
By means of maps and sections it is difficult to represent the 
structure of a very complexly folded district, and even a dissected 
model does not represent it completely, as it is impossible to 
show in true porportion the different orders of folds, and espec- 
ially those of the higher orders. It is plain that cross-sections 
ina single direction at long intervals fail to give any adequate 
idea of the structure of such a district, although these combined 
with geologic and topographic maps may do so. In reports the 
structure can best be represented by combining the geologic and 
topographic maps with two sets of cross-sections made at frequent 
intervals and at.right angles to each set of folds. 
It will be noted that in the foregoing treatment of folds they 
are classified as they occur, no ultimate theory of their origin 
being offered. No conception of the causes of mountain ranges 
enters into the analysis. It is true that an explanation is 
attempted of the difference between normal and abnormal com- 
posite folds. The fact that this explanation apparently accords 
