IMPERFECTION OF GEOLOGICAL RECORD 71 



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-metamorphic") and 

 granitic rocks exceed, in the proportion of 19 to 12-5, 

 the whole of the newer Paleozoic 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 com- 

 pletely 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 

 favorable, as previously explained, for the formation 

 of new varieties and species; but during such peri- 

 ods there will generally be a blank in the geological 

 record. On the other hand, during subsidence, the in- 

 habited area and number of inhabitants will decrease 

 (excepting on the shores of a continent when first broken 

 up into an archipelago), and consequently during subsi- 

 dence, 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. 



