368 On the Geoloy teal Structure of the Amazons Valley. 



I am not prepared to give the vertical or horizontal distri- 

 bution of these fossils. So far as visible at low water, they 

 appear to range over 20 feet of depth, coming nearer to the 

 surface at Pebas than at Iquitos ; but the main layer lies 

 nearly parallel with the level of the river, which falls about 

 40 feet between the two places. They occur on both sides of 

 the lignite, which is traceable from Tabatinga to the Huallaga. 

 The shell-bed must extend far west of Iquitos ; and in my 

 last expedition I procured a mass of yellow clay, containing 

 the " Pebas shells," from a point several hundred miles up the 

 Ucayali : the precise locality I cannot give, as I did not visit 

 it. Evidently this Tertiary basin is not so contracted as the 

 glacialists have tried to made it. Dr. Gait brought from the 

 Pachitea (near the junction of the Pichis and Palcazu) a beau- 

 tiful Ostrea, which Conrad calls 0. callacta, and says it is a 

 Tertiary form, and was filled with a light-coloured clay strikingly 

 similar to that of the Pebas beds*. Mastodon remains have 

 been found near Moyobamba; and silicified wood is occasionally 

 seen in the hands of the Maranon Indians. 



It is evident that such an even sheet of fine earth could not 

 have been spread over such a vast area by streams from the 

 rising Andes ; it must be the deposit of a quiet inland lake. 

 It is evident that the Amazons estuary extended further west 

 than now, the result of a gentle oscillation : a subsidence of 

 one hundred feet at Tabatinga would make the tides felt on 

 the Maranon. It is evident that the condition of things in 

 the Brazilian Amazons, both during and after the deposition of 

 the formation, was different from that in the Maranon region. 

 If there is any difference in age, I should give the priority 

 to the latter. It is evident that the Andes did not reach their 

 present altitude until after the deposition of the Amazonian 

 formation, — though it was a slow movement, in mass ; for the 

 beds are nowhere unequally tilted or dislocated. The clay- 

 beds ascend with gentle inclination the eastern slope, being- 

 visible far up the Napo, Pastassa, and Huallaga. Balsa Puerto, 

 3° 15' west of Iquitos and 400 feet higher, stands on a thick bed 

 of red, yellow, and white clays, resting on a soft slate, dipping 

 easterly f. By the continued rise of the Andes, the great 

 equatorial lake, already shallowed by sediment, was drained, 

 leaving only a network of rivers, igarapes, and lagunes. 



Poughkeepsie, New York, 

 September 22, 1875. 



* Til the ferruginous clay at Villa Bella, Lower Amazons, I found 

 imbedded a little shell, which Conrad refers to Acicula. 



t At the head of the Napo and Pastassa the Andes begin with a soft 

 slate of great thickness, overlying mica-schist and trachyte. 



