THE SUCCESSION OF STRATA. 57 



are said to be conformable to each other. This is 

 particularly true of an area undergoing depres- 

 sion. Yet as depression extends, and an area 

 which had previously been dry land is submerged 

 so as to be covered with new deposits, worn from 

 the higher land which is near to the now sub- 

 merged area, such a new layer begins a new order 

 of succession in that district, and rests upon the 



A many older deposits which had been 

 tilted up and worn level, SO that their edges be- 

 came exposed before the old land was raised from 

 the s L -a. Such a succession is -aid to be uncon- 



There is an 1111: ted interval of 



time between such unconformable strata and the 



m whieh they rest ; but the unconformity 



is local, and does not imply any real break in the 

 On of rocks, for the break is a COnse- 



quence of the subn e of the denuded land, 



which had interrupted the even spread of sedi- 

 ments by the m - it remained above 



the - 



On the other hand, when land is undergoing 

 uphi ind the shore deposits begin to be 



raised out of the water, it must happen that the 



newest formed deposits will be worn tip and re- 

 moved before they enu sea. There 

 is a break formed in this way in the horizontal 

 sequence, though there is no break in the vertical 

 sequenct rata. Traced by their mineral 

 character to the circumstances in which they 

 originate, the whole succession of water-formed 

 rocks which is known demonstrates no more than 

 three or four great oscillations in level of the 

 earth's surface, which have converted lands into 

 seas, and seas into lands. 



It is obvious that land is disturbed in level in 



