352 SLUIDIGES, ITOMR SI ULOMSIN INS, 
folded rocks are really the older and belong to a series prior to 
their unfolded westward continuation. The failure to appreciate 
the above principle has been to a large degree the cause of the 
Taconic controversy. 
It is believed that the cause of the frequent sudden change 
from closely folded to very gently folded rocks across the strike 
of the folds is due to the principle explained on pages 317-318. 
This is: Strata when once bent ata certain place continue to 
bend at this place rather than to form a new fold. This bending 
continues until, as a result of the folding, the strata are greatly 
thickened and the inclinations become steep, so that resistance 
to further folding at this place is greatly increased. The force 
is then transmitted forward and a new area is affected by folding, 
_ but as soon as the strata are here bent they continue to bend 
easily until they are closely folded, so that there is the same sud- 
den transition as before from the closely folded to the very gen- 
tly folded or unfolded districts. 
In the first and simplest case the lower formations have been 
subjected to either simple or complex folding, while the upper 
formations are undisturbed or very slightly disturbed. In this 
case the upper formations are likely to be found as inlyers upon 
the other, and the structural break between the two 1s compara- 
tively easy to determine. Phenomena of this kind are found at 
many localities between the Paleozoic and pre-Paleozoic sedi- 
ments, and less frequently they are found wholly within the pre- 
Palaeozoic formations. 
The second case is that in which the lower formations were 
folded by one or more movements before the upper series was 
deposited, and subsequently the two were again folded. If the. 
second folding was of a comparatively simple character, and the 
earlier was rather complex, it is usually comparatively easy to 
separate the two series. For instance, the lower formations may 
have been rather closely folded by the first orogenic movement, 
and the two sets of formations together may have been gently 
folded by the second movement. The discrepancy between the 
two may often be detected, even when the movements were in 
