GEOLOGY. 271 



Erosion of tTie Coast of Brazil. — He said that there had taken 

 place ou the coast of Brazil the most extraordinary encroachments 

 in the annals of geology, that there had also been great disj^lace- 

 ment in the whole Amazonian vallej', and that all these changes 

 had occurred within a comparatively recent geological period. 

 It had become an axiom in geology that the highest mountains 

 were the most recent ; and, therefore, the table lands of Guiana 

 and high lands of Brazil are older than the Andes. It had 

 been ascertained, by scientific explorations fifteen years ago, 

 that the slopes of the mountains dipping toward the north con- 

 tain deposits belonging to the ti'ansition period. The recent 

 upheaval of the Andes showed that the trough now formed by 

 them was once open to the Pacific. This valley, in its formation, 

 had many of the characteristics of the valley of the Mississii^pi, 

 which was formed, first, by the upheaval of the Laurentian hills 

 to the north, trending east and west ; next, the upheaval of the 

 Alleghanies, trending north and south, and leaving the plain of 

 the Ohio open to the Pacific ; and, last, the upheaval of the Rocky 

 Mountains. It became therefore an interesting question, how far 

 the Mississippi trough was lined with cretaceous deposits. By 

 the discovery of fossil fishes in the Ceara region, its age had been 

 determined as belonging to the cretaceous period. He was free 

 to admit that the basin had its present outline since the cretaceous 

 period, and that whatever was now found within that trough was 

 younger than the cretaceous beds. It had since been filled with 

 deposits before described, and was now losing ground by the 

 euGroachments of the sea. 



He gave the results of his examination of the coasts of Para 

 River, and other localities farther south, proving, by the existence 

 of fragments of peat and stumps of trees on the coasts below 

 highwater mark, that 'solid land once existed where now the sea 

 flows. There was evidence that there was a time when the bay 

 of Narajo, now sixty miles wide, did not exist. Gi'eat changes 

 had taken j^lace even within three years. Encroachment was 

 going on everywhere at the mouth of the Amazon, by the conflict 

 of the tidal currents with the waves of the sea. There could be 

 no doubt that all the islands on the coast owed their existence to 

 this cause, and that the coast was once a continuous line. If so, 

 the question arose, Through what channel did the Amazon then 

 discharge its waters .f* He answered this question by giving his 

 opinion that the river formerly found its way through what is now 

 the island of Narajo. 



In considering the question how far these encroachments had 

 gone, Professor Agg-ssiz stated his belief that the coast formerly 

 extended three hundred miles beyond its present boundaries, from 

 the promontories of Ceara to Eastern Guiana. It was also prob- 

 able that some of the streams, now flowing into the ocean south 

 of the Amazon, were tributaries of that river before the coast was 

 encroached upon by the sea ; and he was inclined to believe tliat 

 the coast in the vicinity of the Orinoco had been similarly denud- 

 ed, and that the continent was once joined with the West India 

 Islands. How far the Gulf of Mexico owed its existence to the 

 same origin was a question for geologists to decide. 



