CUAF. X.] PAL^IONTOLOGICAL COLLECTIONS. 257 



mantle of rock may not have been very thick. Admitting then 

 that gneiss, mica-schist, granite, diorite, &c., were once necessarily 

 covered up, how can we account for the naked and extensive areas 

 of such rocks in many parts of the world, except on the belief 

 that they have subsequently been completely denuded of all over- 

 lying strata? That such extensive areas do exist cannot be 

 doubted : the granitic region of Parime is described by Humboldt 

 as being at least nineteen times as large as Switzerland. South 

 of the Amazon, Boue colours an area composed of rocks of this 

 nature as equal to that of Spain, France, Italy, part of Germany, 

 and the British Islands, all conjoined. This region has not been 

 carefully explored, but from the concurrent testimony of travellers, 

 the granitic area is very large: thus, Von Eschwege gives a 

 detailed section of these rocks, stretching from Rio de Janeiro for 

 260 geographical miles inland in a straight line ; and I travelled 

 for 150 miles in another direction, and saw nothing but granitic 

 rocks. Numerous specimens, collected along the whole coast from 

 near Rio Janeiro to the mouth of the Plata, a distance of 1100 

 geographical miles, were examined by me, and they all belonged 

 to this class. Inland, along the whole northern bank of the Plata 

 I saw, besides modern tertiary beds, only one small patch of 

 slightly metamorphosed reck, 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 esti- 

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

 find that the metamorphic (excluding " the semi-metariiorphic ") 

 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 



