20 STKATIGEAPHICAL GEOLOGY 



land, there is not only an absence of deposition, but there commences 

 an actual destruction of the records previously accumulated. In 

 the first place, as the rising deposit passes through the plane of the 

 sea-level, it is exposed to the erosion of the waves, and if the latest 

 deposits are not very thick, and the process of elevation is not 

 very rapid, the probability is that only small portions will escape 

 destruction. These portions will remain for a time as plains, marsh 

 lands, and raised beaches ; but as elevation continues, they will be 

 brought gradually under the influence of atmospheric agencies, 

 and exposed to continual wear and tear so long as they remain 

 above the sea. 



It follows, therefore, that only deposits of considerable thickness 

 will survive the attacks of destructive agencies, and remain to form 

 a permanent part of the earth's crust, but thick deposits cannot 

 be accumulated in shallow seas, which are the most favourable to life, 

 so long as the sea-bed remains stationary or is being elevated. 

 Thick formations can only be accumulated in areas where constant 

 subsidence has been in progress, or where the shore originally 

 shelved rapidly into deep water, as off the coast of the Riviera. 

 In the latter case the sediment will be rapidly accumulated, and 

 will not contain so many or such varied forms of life ; but under 

 the former conditions, if the supply of sediment was sufficient to 

 keep the sea shallow and to embed the organic remains, a formation 

 rich in fossils, and thick enough, when upraised, to resist erosion, 

 would be formed. We must, therefore, conclude with Darwin, 

 " that nearly all our ancient formations, which are throughout the 

 greater part of their thickness rich in fossils, have thus been formed 

 during subsidence." * 



This conclusion that most of our great fossil-bearing formations 

 have been deposited during subsidence has a very important bearing 

 on the question before us, viz. whether the rocks are likely to con- 

 tain the remains of an uninterrupted succession of species, or whether 

 the record is more or less incomplete ; for, as Darwin remarks : 



" During periods of elevation the area of the land and of the 

 adjoining shoal parts of the sea will be increased, and new stations 

 will often be formed all circumstances favourable, as previously 

 explained, for the formation of new varieties and species ; but 

 during such periods there will generally be a blank in the 

 geological record. On the other hand, during subsidence the 

 inhabited area and number of inhabitants will decrease (excepting 

 on the shores of a continent when first broken up into an archi- 

 pelago), and consequently during subsidence, though there will be 

 much extinction, few new varieties or species will be formed ; 

 1 Origin of Species, sixth edition, p. 273. 



