Amazonian Formation. 563 



ingly similar to that of the Pebas beds.* Mastodon re- 

 mains 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 ex- 

 tended farther west than now, the result of a gentle oscil- 

 lation : a subsidence of one hundred feet at Tabating^a 

 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 lat- 

 ter. It is evident that the Andes did not reach their pres- 

 ent altitude until after the deposition of the Amazonian 

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

 beds ai-e nowhere unequally tilted or dislocated. The clay- 

 beds ascend with gentle inclination the eastern slope, being 

 visible far up the ISTapo, Pastassa, and Hnallaga. 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.! By the continued rise of the 

 Andes, the great equatorial lake, already shallowed by sed- 

 iment, was drained, leaving only a net-work of rivers, iga- 

 rapes and lagunes. 



* In the ferruginous clay at "Villa Bella, Lower Amazons, I found imbed- 

 ded 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. 



