CiiAr. IX. TALEONTOLOGICAL COLLECTIONS. 277 



by Mr. Hopkins, lliat if one part of the area, after rising and 

 before being denuded, subsided, the deposit formed during the 

 rising movement, though not thick, might afterward become 

 protected by fresh accumuhitions, and thus be jireserved for a 

 long period. 



Mr. Hopkins also expresses his belief that 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, Avill admit that 

 tlicse latter rocks have been denuded on an enormous scale. 

 For it is scarcely possible that such rocks could have been 

 solidilied and crystallized while imcovered ; but if the meta- 

 morphic 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., 

 w-ere once necessarily covered up, how can we account for the 

 naked and extensive areas of such rocks in many jiarts of the 

 world, except on the belief that they have subsequently been 

 completely denuded of all overl^ang strata ? lliat such ex- 

 tensive 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 

 colors an area comjiosed of rocks of this nature as equal to tliat 

 of Spain, France, Italy, part of German}-, and the British Isl- 

 ands, all conjoined. This region has not been carefully ex- 

 plored, but, from tlie concurrent testimony of travellers, the 

 granitic area is very large : thus. Von Eschwege gives a de- 

 tailed section of these rocks, stretching from Rio dc Janeiro 

 for 260 geograjihical miles inland in a straight line ; and I trav- 

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

 granitic rocks. Numerous specimens, collected along the whole 

 c(jast from near Rio Janeiro to tlie mouth of the Plata, a dis- 

 tance of 1,100 geographical miles, were examined l)y me, and 

 they all belonged to this class. Inland, along the whole north- 

 ern bank of the Plata, I saw, besides modern tertiary beds, only 

 one small patch of slightly-metamorphosed rock, ^\■hich alone 

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

 scries. Tuining to a well-known region, namely, to the United 

 States and Canada, as shown in Prof. II. I). Rogers's beauti- 

 ful map, I have estimated the areas l)y (Hitting out and weigh- 

 ing the paper, and I find that liie metamorphic (including "the 

 Henii-metamorphic") and granitic rocks exceed, in the i)n)p()rtion 



