612 THE JOURNAL UF GEOLOGY. 
folds would be necessarily most abrupt the nearer they are to 
A” (p. 184). Further on he gives four chief ‘reasons why 
this action should not produce perfectly regular and uniform 
folds: (1) from a variation in thickness of the bed; (2) from 
a want of uniformity in the material or its state of induration ; 
(3) from an inequality in the action of the force upon different 
parts of the line against which it operates; (4) from irregularity 
in the contraction going on beneath the area.” He observed a 
third principle, viz., that by the effect of gravity alone plication 
would be produced in much inclined or tilted clayey layers, 
while the sandy layers unless greatly indurated would settle 
bodily. A fourth principle is stated thus, ‘‘If the material sub- 
jected to lateral pressure be not capable of folding, or only 
partially so, the region operated upon instead of rising into 
a series of elevations would be raised into one or more ridges of 
much greater height.’”’ A fifth principle is that intruded igneous 
rocks or dikes may not be the cause, but are rather ‘‘a concomi- 
A sixth principle 
x” 
tant result of the same general operation. 
is “the folding of strata by subsidence of the plicated region can 
be only of small extent.” Seventh, he says ‘‘the occurrence of 
volcanoes mostly in the neighborhood of the sea is a necessary 
result of these principles.” he eighth principle is that the 
grander geological epochs are the direct. result of more or less 
catastrophic periods which would separate, according to the 
theory, longer periods of comparative quiet, thus forming the 
transition breaks between the great systems. 
While Dana was a consistent uniformitarian, in so far as to 
interpret past phenomena of the earth’s history by the operations 
of forces such as are now in action, he clearly saw the natural 
relations of periods of special disturbance of the strata by the 
reaching of high degrees of tension and their expression in ele- 
vation and fractures along lines of tension, and the more quiet 
periods of chief sedimentation. This principle is better elabo- 
rated in the latest edition of the Manual than in previous works, 
on account of the fuller knowledge of the facts finally attained. 
In the development of the American continent there are recog- 
