UNCONFORMITY 269 



sand, which, consolidating, has resisted weathering better than the 

 shale and so stands out in relief. These are called sandstone 

 dikes, though they are not true dikes, which are of igneous rock. 

 The sand, in this and similar cases, has been forced in by pressure 

 from below while still loose. 



Unconformity 



We have hitherto considered the stratified rocks as made up of 

 beds which follow upon one another in orderly sequence, and are 

 all affected alike by the elevation or depression, folding or disloca- 

 tion, to which they may have been subjected. Strata which have 

 thus been laid down in uninterrupted succession, with sensibly 

 parallel bedding planes, and which have been similarly affected by 

 movements, are said to be conformable, and the structure is called 

 conformity. In many places, however, the strata exposed in a 

 section are very obviously divisible into two groups, each made up 

 of a series of conformable beds, but the upper group, as a whole, 

 is not conformable with the lower, but rests upon its upturned 

 edges, or its eroded surface. The two groups are said to be 

 unconformable and the structure is named unconformity. The 

 definition of unconformity here given includes certain not uncom- 

 mon structures, which must be distinguished as having quite a 

 different significance. 



Unconformity is of two kinds. ( 1) There is a distinct difference 

 in the dip of the two sets of strata, the upper beds lying across the 

 upturned and truncated edges of the lower. This is the more 

 usual kind and is shown in Figs. 117 and 133. The structure im- 

 plies that the lower series, of beds was first laid down under water, 

 and that they were then upturned, tilted or folded to form a land 

 surface. Erosion next truncated the folds, planing the edges of 

 the disturbed beds down to a more or less level surface. The 

 land surface was again depressed beneath the water, and the 

 second set of strata was deposited upon it. Finally, a renewed 

 elevation, accompanied perhaps with folding or faulting, has 

 brought both series of strata above the sea-level. 



While the older beds formed a land surface, they were eroded 



