192 STRUCTURAL GEOLOGY. 



said to be conformable. But if two series are discontinu- 

 ous i. e., separated by an erosion-surface or old land-sur- 

 face, and therefore formed at different times and under 

 different conditions they are said to be unconformal)le. 

 In all the figures the strata of the lower series are con- 

 formable throughout, and so are also those of the upper, 

 but the two series are unconformable with each other, the 

 line of unconformity being an old eroded land-surface. 



Even so simple sections as Fig. 108, one of the com- 

 monest observed, record many interesting events in the 

 history of the earth, viz. : 1. A long period of quiet, 

 during which the first series of strata was deposited. 

 2. A period of commotion, during which the sea-bottom 

 here was elevated into land, and perhaps the strata crum- 

 pled. 3. A long period during which it remained land- 

 surface and was deeply eroded and the strata-edges exposed. 

 4. Another period of commotion, during which it sank 

 again and became sea-bottom. 5. Another long period 

 of quiet, during which the second series of strata was de- 

 posited ; and, 6. Still another period of movement, by 

 which the whole was finally raised and became thus sub- 

 ject to the inspection of the geologist. 



The following diagrams (Fig. 109) represent the man- 

 ner in which the phenomena may be supposed to have 

 occurred. In A, we have thick sediments, Sd, accumu- 

 lated on an off-shore sea-bottom. In B, the same have 

 been elevated into land, and crumpled. In (7, they have 

 been eroded and their edges exposed. In D, they have 

 again subsided beneath the sea, and received sediments, 

 Sd, on their eroded edges. 



Since geological history is mainly recorded in stratified 

 rocks, and since, while a place is land-surface and being 

 eroded, there can be no strata formed there, it is evident 

 that a line of unconformity always indicates a period of 

 which there is no record at that place, although the record 

 may be found elsewhere. Unconformity, therefore, al- 



