

OVERLAP. 79 



eposits on which they there rest. This condition is termed overlap, 

 or transgression, because the overlying deposit extending beyond the 

 beds previously deposited overlaps and covers them up. Overlap 

 occurs whenever the level of land is depressed over a wide area, so 

 as to allow the sea to extend inland and throw down a stratum upon 

 ground where the series had necessarily been interrupted. A remark- 

 able overlap of the Chalk is seen in Yorkshire, for in the cliffs at 

 Speeton the conformity is perfect ; but as the Chalk extends inland it 

 rests successively upon all the Secondary strata down to the Trias. 

 Similarly in Dorsetshire and Wiltshire, the Cretaceous rocks are con- 

 formable to the underlying series, but as they extend westward the 

 Upper Greensand rests successively upon all the Secondary strata, till 

 in the Haldon Hills it overlaps Carboniferous rocks. 



UNCONFORMABLE 



Fig- 33- Diagram of Overlap. 



Principal Epochs of Convulsion. By pursuing this investigation 

 in different situations, we find that these internal movements or con- 

 vulsions happened at intervals during the whole period of time occu- 

 pied in the deposition of the strata. Some of the most prevalent and 

 remarkable cases of dislocation and unconformity are, however, obser- 

 vable : (i) immediately after the deposition of the Cambrian series, 

 between the Upper and Lower Llandovery beds ; (2) after the accumu- 

 lation of the Coal Measures in the Carboniferous system ; (3) after the 

 deposition of the oolitic rocks ; (4) after the deposition of the Chalk ; 

 and (5) one of the most recent probably of all, after the completion of 

 all the regular formations above the Chalk. It is not to be supposed 

 that all even of these principal cases of dislocation can be recognised 

 in every country ; on the contrary, the subterranean forces frequently 

 shifted their directions and points of action. There is no difference 

 except in magnitude, and the degree to which it is deep seated between 

 the displacement of surface indicated by an ordinary fault and the 

 submergence or elevation of the largest areas of land. 



We shall have occasion to show, while speaking of the organic 

 remains, that there is sometimes observed a singular harmony between 

 these periods of extraordinary internal disturbance and the several 

 epochs when the different races of animals and plants came into exist- 

 ence ; and it is not unreasonable to suppose, that in this manner we 

 may find it possible to establish such a relation between physical and 

 organic phenomena as to demonstrate the geological dependence of the 

 distribution and mutations of life upon changes taking place in the 

 earth's physical geography in successive ages. 



Proximity of Mountains. At present, restricting ourselves to the 

 phenomena of elevation and disruption of the strata, we shall carry our 

 inductions one step further, for the purpose of proving what was before 



