180 



STRATIFIED OR SEDIMENTARY ROCKS. 



strata-edges exposed, to sink down and become ocean-bottom, and re- 

 ceive sediments covering the strata-edges and filling the erosion-hol- 

 lows, and afterward to rise again and be submitted to the inspection of 

 the geologist; Fig. 157 represents in section what he would see. They 



are interpreted as 

 A 



follows : In A 

 and B the lower 

 series of strata 

 was first deposit- 

 ed ; then the sea- 

 bottom was raised 

 to land - surface 

 and the strata 

 tilted and eroded ; 

 then it went down 

 again and re- 

 ceived the upper 

 series ; and, final- 

 ly, was raised and 

 inspected by the 

 geologist. In C 

 the process was 

 the same, except 

 that the first se- 

 ries of strata was raised without tilting. In D the second series of 

 strata ivas also tilted in the second raising. Now the strata of each 

 series are said to be conformable among themselves, but the two series 

 are unconformable with one another. 



Definition. — Therefore, strata are said to be conformable when they 

 are parallel, continuous, and therefore formed under the same condi- 

 tions, and are unconformable when they are discontinuous, and formed 

 under different conditions ; the discontinuity being ahvays marked by 

 an old eroded land-surface. Unconformable strata are usually non- 

 parallel, and this is often made a part of the definition ; but this is not 

 necessary. In Fig. 157, C, there is no want of parallelism. The reason 

 we have already explained. 



A section like any one of the foregoing — among the commonest in 

 geology — reveals many interesting events : 1. A long period of quiet, 

 during which sediments of the first series were deposited continuously 

 on a sea-bottom. The length of this period is measured by the thick- 

 ness of the series. 2. A period of elevation, during which the sea- 

 bottom became land-surface. We have no means of estimating the 

 length of this period. 3. A long period during which the land-surface 

 became deeply eroded. The length of this period is measured by the 



Fig. 157.— Some Cases of Unconformity. 



