TIFF, ANNALS ^ 



MAGAZINE OF NAirUAL HISTORY. 



[FOURTJI SERIES.] 

 No. 47. XUVEMBER 1S71. 



XXXVI. — On the Evidence of a Glacial Epoch at the Equator. 

 By James Orton, of Poughkeepsie, N. Y.* 



The valley of the Amazon is liiglily interesting to the geolo- 

 gist, from its vast extent and its disputed origin. Probably 

 no other region on the globe, of equal area, has such a re- 

 markably uniform character : from the Andes to the Atlantic, 

 and from the tails of the ^Madeira to the Oi-inoco, scarcely 

 any thing is visible but clays and sandstones f. Professor 

 Agassiz was the first geologist of eminence to explore any 

 considerable part of the fonnation. He ascended the river to 

 Tabatinga (loOO miles in a straight line) ; and he has well 

 described the successive beds, of which he distinguislies ten. 

 The chief, in the order of superposition, are : — coarse sand, 

 laminated clays of divers colours, ferruginous sandstone, and 

 an unstratitied sandy clay ; of these, the argillaceous portion 

 is the most important, as it is the most extensive, the sand- 

 stone being reduced to isolated hills by denudation. The 

 clays generally are very fine in texture, and without a pebble : 

 they contain a large percentage of iron, but no trace of lime ; 

 there are, however, calcareous concretions, nodular or stalac- 

 tiform, strikingly similar to the marly concretions noticed by 

 Darwin in the Pampean mud. The argillaceous dejjosits are 

 more conspicuous on the Upper Amazon, and the sandstones 

 on the Lower. The whole formation dips gently to the east, 

 and its total thickness is about 800 feet. 



Professor Agassiz considers the valley a cretaceous basin, 

 filled with glacial drift — in other words, that all these clays 



* From a separate impression communicated by the Author. 



t Professor Agassiz speaks of this clay formation as stretchinpr over a 

 surface more than three thousand miles in length ; but he is evidently 

 led astray by the length of the Amazon, with all its windings. Tlie 

 width of the continent at the equator is only 2,100 miles. 



Ami. <&Ma(/.N. Hist. Ser. 4. TW. viii. 2;^ 



