Professor Agassiz in Brazil 39 



it was not long before he found, at Tijuca, "a drift-hill with in- 

 numerable erratic boulders, as characteristic as any he had seen in 

 New England." The sandy clay of the vicinity which rested 

 immediately upon the partially stratified metamorphic rock fre- 

 quently contained these boulders. The extensive decomposition 

 of the rock, which often made it very difficult to determine the line 

 of demarcation between the rocks and the drift, had, of course, 

 obliterated every trace of rock-polishing or grooving which might 

 once have been present ; yet, even where the disintegration was 

 extensive, it had not destroyed the undulating lines of the rochcs 

 nioutouiih upon which the drift rested. Aided by frequent examina- 

 tions of the geology in the vicinity of Rio, Mr Agassiz and his 

 assistants were able to trace the same formation throughout every 

 part of Brazil which they explored. Whatever the nature of the 

 underlying rock, they always found at the surface the same homo- 

 geneous clayey reddish paste containing quartz pebbles. In the 

 Amazons valley, however, the relations of this sandy clay to the 

 underlying deposits were of a different nature, and have furnished 

 the basis of a remarkable hypothesis presented pretty fully in this 

 work. Throughout the valley, three distinct deposits occur : 



\st. Finely laminated clays, resting upon a well-stratified sand- 

 stone, and overtopped by a crust almost resembling a ferruginous 

 quartzite. 



2d. A cross-stratified, highly ferruginous sandstone, with occa- 

 sional quartz pebbles. 



3^/. The ochraceous, unstratified sandy clay already mentioned, 

 spreading over the undulating surface of the sandstone, and filling 

 all its depressions and cracks. 



We are not told the extent of the first series of beds ; the second 

 sometimes attains a thickness of more than 800 feet, and the third 

 ordinarily varies from 20 to 50 feet. 



Notwithstanding the thickness of these deposits, their compact 

 structure, and the fact that the first and second are conformable 

 to each other, while the third lies uncomformably above them, Mr 

 Agassiz believes that they all belong to the glacial epoch. He 

 attributes their position and variable character to the conditions 

 under which they were deposited, and the hardness and compact- 

 ness of many in the series, to the heat of a tropical sun. This 

 conviction is founded on the correspondence of their materials to 

 those accumulated in glacier bottoms ; on the resemblance of the 

 uppermost layer to the Rio drift (the glacial origin of the latter he 



