330 SL QIDIVES SHON SL GLOMEIN TES, 
clear that gravity must be an important force, which may greatly 
modify the forms of folds. The particular form of fold in a 
given case is of course the resultant of all the forces which work 
upon the rock stratum composing it. 
So far as I am aware, Dana,' in 1847, was the first geologist 
to call attention to the principle that folds may be modified by 
the force of gravity. As is well known, this idea has been 
recently emphasized by Reyer. 
Origin of abnormal folds.—I1n the abnormal anticlinorium and 
synclinorium new factors enter to modify the result. The first 
is readjustment between the beds. Fig. 15 represents a draw- 
DUI YAN 
Fic. 15.— Representation of simple symmetrical folds, with their axial planes drawn 
on the ends of a bunch of smooth paper three-fourths of an inch thick. 
ing of a number of upright folds made upon the ends of a bunch 
of smooth sheets of paper three-fourths of an inch thick. The 
sheets may be taken to represent thin beds in a nearly homo- 
geneous rock. Fig. 16 represents this same drawing as it was 
distorted when the bunch of paper was folded into anticlines 
and synclines between blocks of wood. It will be seen that, con- 
sequent upon the readjustment of the sheets over one another, 
rendered necessary by the folding, the secondary folds at the 
crests and the troughs remain upright, although compressed if a 
secondary anticline or syncline corresponds with a primary fold 
of the same kind, and dilated if a secondary anticline or syncline 
corresponds with a fold of the opposite kind, and vice versa. 
If the secondary folds were slight, the opening might go so far 
as to obliterate them and the only remaining effect be to flatten 
the primary anticlines or synclines. The secondary folds on 
the limbs of the primary folds are distorted. The readjustment 
therefore mainly affected the forms of the fold upon the limbs. 
Taking as their axial planes the radial planes of the primary 
* Geological results of the earth’s contraction in consequence of cooling, by JAMES 
D. Dana, Am. Jour. Sci., 2d ser., Vol. III, p. 185, 1847. 
