GEOLOGY OF THE LOWER AMAZON REGION 723 



northern and eastern portions of the state of Para represent a very 

 ancient continent, and probably formed the margin of the Paleozoic 

 seas. This land extending across the broad and deeply eroded 

 valley at the mouth of the Amazon, and uniting with the Archean 

 mountains of the state of Ceara and east Brazil, apparently continued 

 to exist in younger Tertiary time. The above hypothesis, therefore, 

 cannot be true for pre-Neogenic time, because across the mouth of 

 the present Amazon there then stood a highland uniting Guiana with 

 the present highland of Ceara. 



This old land is folded. West of Para the folds strike southwest- 

 northeast; farther east, in the region of the Serra Tumuc-Humac, 

 nearly east and west; and along the Atlantic, northwest-southeast. 

 This bowlike trend of the basal mountains continues northward into 

 the Orinoco lowland; in fact, into the Caribbean Sea, while to the 

 southeast of the mouth of the Amazon they extend along the Atlantic 

 coast into the state of Ceara. 



This folding took place before the metamorphic sedimentaries 

 underlying the Siluric originated, the exact age of which is 

 unknown. In their petrographic habit they closely resemble the 

 Archean, but are separated from it by a discordance, while above 

 they seem to grade without disturbance into Siluric strata. 



According to the present distribution of the Paleozoic deposits, the 

 sea was open to the west, as the younger formations occur with great 

 regularity farther and farther away from the Archean. The Carbonic 

 apparently attains the Parii, the Devonic probably continues to 

 Maraca, and the Siluric possibly reaches the Araguari. 



The Siluric and Devonic fossiliferous deposits of the lower Amazon 

 region are tolerably coarse- clastic in character, and undoubtedly 

 were laid down in a shallow sea. The Devonic faunas are very similar 

 to those of North America, and this is all the more remarkable when 

 one considers the great distances by which they are separated. This 

 leads to the conclusion that this old Paleozoic sea had free communi- 

 cation between North and South America. 



As no young Middle and Upper Devonic deposits are known in the 

 Amazon region, it appears that beginning with that time great changes 

 took place in the distribution of land and water. This seemingly 

 connects, on the one hand, with the outbreaks of eruptive rocks 



