Chap. X.] PAL^ONTOLOGICAL COLLECTIONS. 65 



small patch of slightly metamorphosed rock, which 

 alone could have formed a part of the original capping 

 of the granitic series. Turning to a well-known region, 

 namely, to the United States and Canada, as shown in 

 Professor H. D. Eogers's beautiful map, I have estimated 

 the areas by cutting out and weighing the paper, and I 

 find that the metamorphic (excluding " the semi-meta- 

 " morphic ") and granitic rocks exceed, in the proportion 

 of 19 to 12 -5, the whole of the newer Palaeozoic 

 formations. In many regions the metamorphic and 

 granitic rocks would be found much more widely 

 extended than they appear to be, if all the sedimentary 

 beds were removed which rest unconformably on them, 

 and which could not have formed part of the original 

 mantle under which they were crystallized. Hence it 

 is probable that in some parts of the world whole 

 formations have been completely denuded, with not a 

 wreck left behind. 



One remark is here worth a passing notice. 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 archipelago), and consequently during subsidence, 

 though there will be much extinction, few new varieties 

 or species will be formed ; and it is during these very 

 periods of subsidence, that the deposits which are 

 richest in fossils have been accumulated. 



