80 
ELEMENTS OF GEOLOGY. 
months, or even years, will sometimes elapse between the 
first bending of the pavement and the time of its reaching 
the roof Where the movement has been most rapid, the 
curvature of the beds is most regular, and the reunion of the 
fractured ends most complete; whereas the signs of dis¬ 
placement or violence are greatest in those creeps wliich 
have required months or years for their entire accomplish¬ 
ment. Hence w^e may conclude that similar changes may 
have been wrought on a larger scale in the earth’s crust by 
partial and gradual subsidences, especially where the ground 
has been undermined throughout long periods of time; and 
we must be on our guard against inferring sudden violence, 
simply because the distortion of the beds is excessive. 
Engineers are familiar with the fact that when they raise 
the level of a railway by heaping stone or gravel on a foun¬ 
dation of marsh, quicksand, or other yielding formation, the 
new mound often sinks for a time as fast as they attempt to 
elevate it; when they have persevered so as to overcome 
this difficulty, they frequently find that some of the adjoin¬ 
ing flexible ground has risen up in one or more parallel 
arches or folds, showing that the vertical pressure of the 
sinking materials has given rise to a lateral folding move¬ 
ment. 
In like manner, in the interior of the earth, the solid parts 
of the earth’s crust may sometimes, as before mentioned, be 
made to expand by heat, or may be pressed by the force of 
steam against flexible strata loaded with a great weight of 
incumbent rocks. In this case the yielding mass, squeezed, 
but unable to overcome the resistance which it meets with 
in a vertical direction, may be gradually relieved by lateral 
folding. 
Dip and Strike.—-In describing the manner in which strata 
depart from their original horizontality,some technical terms, 
such as “dip” and “strike,” “anticlinal” and “synclinal” 
line or axis, are used by geologists. I shall now proceed to 
explain some of these to the student. If a stratum or bed 
of rock, instead of being quite level, be inclined to one side, 
it is said to dip; the point of 
___N the compass to which it is in¬ 
clined is called the point of 
dip^ and the degree of devia¬ 
tion from a level or horizontal 
line is called the amount of 
dip^ or the angle of dip, Th u s, 
in the annexed diagram (Fig. 60), a series of strata are in¬ 
clined, and they dip to the north at an angle of forty-five 
