Stratification Planes. — Keyes. 299 
Unconformities represent, for the region in which they oc- 
cur, epochs or periods during which erosion, instead of deposi- 
tion, has taken place. They may be very local in extent, but 
usually they indicate the existence of conditions that are wide- 
spread. The fact that the lower set of beds is more or less tilted 
and their upturned edges worn ofif implies a considerable 
elapse of time. Mountains may have been formed and re- 
moved in the interval between the formation of the two sets of 
rocks. 
As a rule the discordance in dip is not the only notable 
difference between the sequence above and below the plane of 
unconformity. Besides being merely tilted, the beds below are 
often folded, metamorphosed and perhaps intersected by erup- 
tive masses. 
The unconformity planes, inconspicuous and as dif^cult to 
trace as they are. have, then, an importance that has rarely 
been appreciated to the extent that it really deserves. They in- 
dicate, as it were, halting places that enable geological history 
to be subdivided and the rock formations to be classified in the 
same way that great national catastrophes mark epochs in hu- 
man history. 
Unconformities are generally considered as local phenom- 
ena. This idea does not always take into consideration the real 
nature of the facts. Very marked discordance is liable to be 
confined to limited belts. The minor features of unconformi- 
ties are far more extensive. As Van Hise has remarked, an 
orogenic movement may relieve itself along a comparatively 
narrow belt, that as a result of differential uplift is one of great 
denudation, while the adjoining plain is little folded and little 
eroded. The stratification of the new series would nearly co- 
incide with those of the older in the plain, Ijut the two would 
meet at high angles in the disturbed belt. 
To recapitulate, it would appear that: 
1. The so-called bedding planes usually recognized as 
characterizing the sedimentary rocks are in reality complex 
])henomena instead of simple; 
2. Bedding planes, so-called, are usefully grouped into 
several distinct classes according to their historical signifi- 
■cance. 
3. The variable value of the so-called bedding planes is 
