ed 
ars | 
cessively on different parts of the older mass. This relation is 
termed an unconformability. The upper conformable beds (6 ¢ d) 
are said to lie unconformably upon the lower (a @). 3 
It is evident that this structure may occur in ordinary sedi- 
mentary, igneous, or metamorphic rocks, or between any two of 
these great series. It is most familiarly displayed among clastic 
formations, and can there be most satisfactorily studied, since the 
lines of bedding furnish a ready means of detecting differences of in- 
clination and discordance of superposition. But even among igneous 
protrusions and in ancient metamorphic masses, distinct evidence of 
unconformability is not always difficult to trace. Wherever one 
series of rocks is found to rest upon a highly denuded surface of an 
older series, the junction is unconformable.’’ Hence, an uneven 
irregularly-worn platform below a succession of mutually conform- 
able rocks is one of the most characteristic features of this kind of 
structure. . 
It has already been pointed out, that though conformable rocks 
may usually be presumed to haye followed each other continuously 
without any great disturbance of geographical conditions, we cannot 
always be safe in such an inference. But an unconformability leaves 
no room to doubt that it marks a decided break in the continuity of 
deposit. Hence no kind of geological structure is of higher import- — 
ance in the interpretation of the history of the stratified formations 
of a country. In rare cases an unconformability may occur between 
two horizontal groups of strata. On the left side of Fig. 311, for 
instance, the beds d follow horizontally upon the horizontal beds (a). 
Were merely a limited section visible disclosing only this relation of 
the rocks, the two groups aand d might be mistaken for conformable 
portions of one continuous series. Further examination, however, 
would lead to the detection of evidence that the limestone a had 
600 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. | 
been upraised and unequally denuded before the deposition of the . 
overlying strata bed. This denudation would show that the apparent 
conformability was accidental, that the older rock had really been 
upraised and worn down before the formation of the newer. In such 
a case the upheaval must have been so uniform over some tracts as 
not to disturb the horizontality of the lower strata. 
As arule, however, it seldom happens that movements of this 
kind have taken place over an extensive area so equably as not to 
produce a want of coincidence somewhere between the older and 
newer rocks. Most frequently the older formations have been tilted 
at various angles, or even placed on end. ‘They have likewise been 
irregularly and often enormously worn down. Hence, instead of 
lying parallel, the younger beds run transgressively across the 
upturned denuded ends of the older. The greater the disturbance of 
the older rocks the more marked is the unconformability. In 
' The occurrence of considerable contemporancous erosion between undoubtedly 
conformable strata belonging to one continuous geological series has already (p. 480) 
been described. 
