GENERAL AND ECONOMIC GEOLOGY 229 



Our knowledge of the geology of the State of Amazonas is of the frag- 

 mentary kind found in the notes of travelers along the Amazon, the Eio 

 Negro, and the Madeira rivers and a few other of the large tributaries. 

 Explorations in the region have been confined almost exclusively to trips 

 made along the navigable streams, for there are very few roads in that 

 State. The geology as shown along the extreme northern part of the map 

 is taken from Brown and Sawkins' geological map of British Guiana and 

 from the notes of Eoderic Crandall, who lived for some time at Boa Vista, 

 on Eio Branco. 



A good resume of the geology and physical features is given by Her- 

 bert H. Smith as an appendix to his "Brazil, the Amazons and the Coast'^ 

 (pages 619-635, New York, 1879). Bates' "The Naturalist on the Ama- 

 zon" is a classic, though it does not contain much on the geology. Of the 

 titles given below, the following relate to paleontology: Boettger, Brown, 

 Clarke, Conrad, Ethridge, Gabb, Gervais, Gtirich, and Woodward. 



General geology. — There are two large areas of Archean rocks in the 

 State of Amazonas, one on the north side, the other on the south side of 

 the Amazon Eiver. A great synclinal fold, beginning somewhere in the 

 State of Para, runs along the axis of the Amazon Valley at least as far as 

 Manaus, and in this basin are sedimentary rocks of Silurian, Devonian, 

 and Carboniferous age, all of them dipping gently toward the axis of the 

 valley. 



The Silurian rocks are marine sediments, mostly thin-bedded sand- 

 stones, the equivalent of the Niagara of North America. On the Eio 

 Trombetas they are best exposed at the first and second falls, in zones 

 from six to eight kilometers wide. They are estimated to have a thick- 

 ness of about three hundred meters. 



The Devonian rocks of the state are coarse white and yellow sandstones 

 and black and reddish shales, all of them dipping southward at an angle 

 of five degrees. They have a total thickness of about two hundred meters. 



The Carboniferous beds are exposed on the Jamunda and Uatuma on 

 the north side of the Amazon and on Eio Abacaxis on the south side. 

 They are shales, sandstones, and limestones, the last named containing 

 marine fossils. The total thickness of the Carboniferous beds is about 

 six hundred meters. 



In 1918 the federal government of Brazil had a well put down in the 

 Carboniferous beds on the headwaters of Eio Maues between Tapajos and 

 the Canuma. It passed through limestones, shales, and sandstones, and 

 reached a depth of 292 meters. To a depth of 217 meters several of the 

 limestone beds contained marine Carboniferous fossils. 



These Paleozoic rocks, however, are known only in the eastern end of 



