GEOLOGY. 243 



present, admitting of the accumulation of vast bodies of ice on the 

 summit of the mountains. The valley of the South Platte, as 

 that stream flows through the range east of the South Park, show 

 not only these accumulations of very coarse boulder drift, but, 

 when this drift is stripped off, the underlying rocks are found 

 smoothed and in some instances scratched as if by floating ice- 

 bergs. F. V. Hay den, in the Preliminary Field-Report of the 

 U. S. Geological Survey of Colorado and New Mexico, Washington, 

 1869, page 87. 



VALLEY OF THE RIO GRANDE. 



The broad intermediate space between the range of mountains 

 which form the east side of the Valley of the Rio Grande and the 

 Sierra Madre a main range of the Rocky Mountains, which 

 gives origin to the waters of the Pacific streams is covered 

 with rounded hills, detached ranges, etc., all of which are basal- 

 tic. The two rounded hills, which are very marked, situated 

 nearly opposite to each other on opposite sides of the Rio Grande, 

 Cerro de la Utas, and Cerro San Antonio, are, it seems to me, 

 old craters or fissures out of which issued the melted material 

 which overflowed the sides, and time has weathered the whole 

 mass into its present beautiful rounded form. At this time they 

 look like gigantic mammae. 



I am inclined to regard the Valley of the Rio Grande as one 

 great crater, including within its rim a vast number of smaller 

 craters and dikes, out of which has been poured at some time a 

 continuous sheet or mass of melted material. All the valleys, 

 small and great, give evidence that they have been worn out of 

 this vast mesa. The Rio Grande, from its source in the San 

 Juan Mountains to Albuquerque, flows along its banks through 

 basaltic rocks to a greater or less extent, and as we go northward 

 from it these rocks disappear in part. F. V. Eayden, ibid., p. 72. 



GEOLOGY OF THE VALLEY OF THE AMAZONS. 



The following paragraph, with reference to the sandstones and 

 clays of the Valley of the Amazons,* is taken from Agassiz' 

 Journey in Brazil : 



" The question now arises, How were these vast deposits 

 formed ? The easiest answer, and the one which most readily 

 suggests itself, is that of a submersion of the continent at success- 

 ive periods to allow the accumulation of these materials, and its 

 subsequent elevation. I reject this explanation for the simple 

 reason that the deposits show no sign whatever of a marine origin. 

 No sea-shells, nor remains of any marine animal, have as yet been 

 found throughout their whole extent over a region several thou- 



* For the views of Prof. Agassiz on the Geological History of the Valley of 

 the Amazons see the Journey in Brazil, by Prof, and Mrs. Agassiz, Boston, 

 1868, p. 398 and following. See, also, "Annual of Scientific Discovery," 1867, p. 



270 and following. 



