70 THE ORIGIN OF SPECIES 



For it is scarcely possible that such rocks could have 

 been solidified and crystallized while uncovered; but if 

 the metamorphic action occurred at profound depths of 

 the ocean, the former protecting mantle of rock may 

 not have been very thick. Admitting then that gneiss, 

 mica-schist, granite, diorite, etc., were once necessarily 

 covered up, how can we account for the naked and ex- 

 tensive areas of such rocks in many parts of the world, 

 except on the belief that they have subsequently been 

 completely denuded of all overlying 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, Bou^ colors 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, Yon Eschwege gives a detailed sec- 

 tion of these rocks, stretching from Eio 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, col- 

 lected along the whole coast from near Rio Janeiro to 

 the mouth of the Plata, a distance of 1,100 geographi- 

 cal miles, were examined by me, and they all belonged to 

 this class. Inland, along the wbole northern bank of the 

 Plata, I saw, besides modern tertiary beds, only one small 

 patch of slightly metamorphosed rock, which alone could 

 have formed a part of the original capping of the granitic j 

 series. Turning to a well-known region, namely, to the! 

 United States and Canada, as shown in Professor H. D. 



