2;4 Denudation of granitic Areas. Chap. x. 



the deposit be much consolidated, nor be capped by overlying for- 

 mations, so that it would run a good chance of being worn away by 

 atmospheric degradation and by the action of the sea during sub- 

 sequent oscillations of level. It has, however, been suggested by 

 Mr Hopkins, that if one part of the area, after rising and before 

 bein* denuded, subsided, the deposit formed during the rising move- 

 ment, though not thick, might afterwards become protected by fresh 

 accumulations, and thus be preserved for a long period. 



Mr. Hopkins also expresses his belief tbat sedimentary beds of 

 considerable horizontal extent have rarely been completely destroyed. 

 But all geologists, excepting the few who believe that our present 

 metamorphic^schists and plutonic rocks once formed the primordial 

 nucleus of the globe, will admit that these latter rocks have been 

 stript of their covering to an enormous extent. For it is scarcely 

 possible that such rocks could have been solidified and crystallized 

 whilst uncovered ; but if the metamorphic action occurred at pro- 

 found depths of the ocean, the former protecting 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 sub- 

 sequently 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, 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 con- 

 joined. 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, col- 

 lected 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 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-metamorphic ") 



