MOUNTAINS 321 
This mountain is then worn by denudation for a while, 
and next is partially submerged in the sea, so that on 
its slopes other strata are deposited. In these are en- 
tombed fossils of a later age, while the unconformity 
represented by the denudation, marks a gap in the 
record of the animal life. Folding has again occurred, 
and both the first and second rocks have been uplifted 
and changed in position. Sometimes there are several 
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[Vols FN 
Two sections to show unconformities. The lines AB mark planes along which 
erosion occurred after the lower rocks were formed, and before the upper 
series were deposited. 
such unconformities, the breaks being of different 
lengths. This kind of evidence is sufficiently common 
among mountains to make it certain that mtermittent 
growth is a feature of their history. 
Phenomena accompanying Mountain Growth. — Since 
slipping and faulting are common results of the uplift 
of rocks, mountain development is accompanied by 
jars which are known as earthquakes (p. 553). Again, 
Y 
