332 SIMGIDIES EOI SI (QUOIEIS TAS 
single one in it, anticlinoria might have the axial planes of the 
secondary folds converge upward and diverge downward, and 
synclinoria might have the axial planes of the secondary folds 
diverge upward and converge downward, and thus both become 
abnormal. This readjustment along the beds, as explained in 
my paper in the following number, may in many cases be con- 
sidered as movements along shearing planes. 
The second new factor in the production of abnormal folds 
is the great strength of the older rocks. For a given region, 
upon the average, rocks become stronger with increase of age. 
There are innumerable exceptions to this if too small portions 
of geological time be compared, as period with period, but com- 
paring era with era such exceptions are rare or altogether absent. 
-The Archean rocks are usually stronger than the Proterozoic, the 
Proterozoic rocks are stronger than the Paleozoic, the Paleozoic 
rocks are stronger than the Mesozoic, and the Mesozoic rocks 
are stronger than the Cenozoic. This in the sedimentary strata 
is due to the indurating effects of various geological forces. In 
mountain ranges, where complex anticlinoria and synclinoria 
mostly occur, a great thickness of strata is concerned in the 
major folds, in most cases more than the deposits of an era; so 
that upon the whole in great mountain masses the lower groups 
of rocks are stronger. 
The third cause of the production of abnormal folds may be 
decreasing lateral stress with increasing depth. That such vari- 
ation in stress is a general fact must be true if the theory of the 
level of no lateral stress at a moderate depth be correct. It has 
been pointed out (pages 210-212) that folds must die out with 
increasing depth unless there is great rearrangement of material. 
If it be supposed that the opposing stresses upon opposite sides 
of an anticline or syncline decrease with depth, there will cer- 
tainly be more decided folding of the higher strata than of the 
lower. This implies upward differential movement of a higher 
stratum as compared with a lower beyond that required for nor- 
mal readjustment. (See p. 331). Consequent upon this there 
will be a tendency for the axial planes of secondary folds on 
