2^0 



UNCONFORMITY 



and no deposition took place upon them. Consequently, between 

 the two sets of strata is a gap, unrecorded by sedimentation (at 

 that point), the length of which represents the time that the older 

 beds were above water. The processes involved in an unconform- 

 ity are of slow operation, so that the gap usually implies a very 

 long lapse of time. In many cases whole geological ages, of in- 



Fig. 117. 



•Unconformity. Diagrammatic section through the strata seen in 

 Fig. 133. P- 3 J 7- 



calculable duration, have intervened between the deposition of the 

 two groups of strata. 



(2) In the second kind of unconformity the two groups of 

 strata have the same dip, the upper series resting upon the eroded 

 surfaces of the lower. The processes involved in this kind of un- 

 conformity are nearly the same as in the first, so far, at least, as the 

 alternation of land surface and sea-bottom, elevation and depres- 

 sion, are concerned. In this case, however, the first upheaval was 

 not accompanied by any folding or fracturing of the beds. ■ An 

 unconformity of the second class is sometimes exceedingly diffi- 

 cult to detect and then is called a deceptive conformity. Such a 



