Chap. X.] PAL^ONTOLOQICAL COLLECTIONS. 65 



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. Rogers'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 propor- 

 tion 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 ex- 

 tended 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 crystalUzed. Hence it 

 is probable that in some parts of the world whole forma- 

 tions 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 ad- 

 joining shoal parts of the sea will be increased, and new 

 stations will often be formed: — all circumstances favour- 

 able, 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 decreasie (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 varie- 

 ties 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. 



