a Glacial Efoch at the Equator, 303 



4° *. But between Pebas and Paid, a distance of 1600 miles, 

 the slope is only 8' 5", or about 2^ inches per mile ; and from 

 the tip-top of the Andes to the Atlantic the inclination is 

 6' 30". We conclude, therefore, that if a sheet of ice ever 

 spread from Cotopaxi to the mouth of the Amazon, it remained 

 there, immovable as the mountains. 



But difficulties lie beyond this. As the length of a glacier 

 depends greatly upon the speed with which it travels, it 

 will be short in proportion as the angle of the slope is dimi- 

 nished. And, further, suppose the ice-sheet formed and 

 moving, what would be its flow ? Even if its rate equalled 

 that of the Mer de Glace, a boulder from the Andes would be 

 over 20,000 years in reaching the Atlantic; but when we 

 consider its feeble slope, and its retardation by the constant 

 trade-winds, we may wonder if it ever completed its journey. 

 Yet this Agassiz glacier is represented as doing a greater 

 amount of work than the high-latitude glaciers, grinding up 

 and covering the vast basin with 800 feet of detritus, " the 

 most colossal drift formation known." And, again, all the 

 slope of any consequence lies between the axis of the Andes 

 and Pebas, a distance of 450 miles. In this abrupt descent 

 (35 feet per mile) it must receive momentum to carry it over 

 an almost level plain of 1600 miles. Why did it not plough 

 up the silt, creating linear lakes like Como and Maggiore, 

 which radiate at right angles to the strike of the Alps ? Yet 

 there is no appearance of excavation. The lagunes of the 

 Napo are shallow ponds. 



4. The existence of such a continental glacier at the equator 

 would profoundly affect the life-history of the globe. As 

 Newberry says, "" Nearly all the fossil plants and mollusks of 

 the strata deposited immediately anterior to the glacial epoch 

 are undistinguishable from species now living in the same 

 region" f. If a mantle of ice ever covered Amazonia, un- 

 doubtedly it had lateral branches descending the valleys of 

 the Orinoco and Paraguay : there is a close similarity of the 

 formation in these valleys to the Amazonian clay, which has 

 resulted, we think, from a contemporaneousness, if not identity, 

 of origin ; and so low is the watershed, especially on the north, 

 that the two river-systems are joined by natural canals f. 



* The average slope of the Mer de Glace is 14°, that of the Greenland 

 glacier 11°. 



t In the opinion of De CandoUe, subscribed to by Gray as likely, the 

 greater part of the existing species of plants are older than the present 

 coniifiuratiou of our continent. 



\ The Casiquiari is only 400 feet above the sea, or about 200 above the 

 centre of the Amazon basin. 



