466 R. Raihhun — Geology of the Lower Amazonas. 



beds of the two series of the metamorphic deposits, but the evi- 

 dence goes to prove that the older, or Laurentian, had been 

 more or less disturbed and metamorphosed, before the deposi- 

 tion of the newer, although the great general movement of up- 

 heaval, that affected, and gave character to, the entire meta- 

 morphic region of Brazil, was posterior to both. 



The formations above the metamorphic, so far observed in 

 the Lower Amazonian valley, are the Upper Silurian, Devo- 

 nian, Carboniferous, Cretaceous and Tertiary. The U])per Silu- 

 rian immediately follows the metamorphic series, on the north 

 side of the vallev, but has not yet been recognized to the south 

 of the Amazonas. On the Rios Trombetas, Curua and Mae- 

 cuni, where they were examined by Mr. Derby and his party, 

 the rocks of this formation are exposed over an area of only a 

 few miles in width, have an estimated thickness of about 1,000 

 feet, and rest upon felsite and syenite ; they are very gently in- 

 clined, and consist mostly of thin-bedded, argillaceous and 

 micaceous sandstones, with some massive beds of pure sand- 

 stone. In the lower part of the series, on the Trombetas, are 

 fossiliferous beds, containing in addition to other species, Ar- 

 throphycus Harlani Hall, Lingula cunenta Con., Orthis hyhrida 

 Sow. and Bucania trilobata Con., which indicate an horizon cor- 

 responding to the Medina Sandstone of the Niagara group of 

 North America. 



The Devonian rocks occupy a broader superficial area than 

 the Upper Silurian, but, so far as studied, are of less thickness, — 

 about 530 feet. They have been traced northward from Erer^, 

 where they were first discovered by Prof. Hartt, in 1870, a dis- 

 tance of about seventy-five miles, on the Rios Maecuru and 

 Curua. Three sections or groups were readily distinguished, 

 differing from one another, both in lithological characters and 

 in fossils. The lower, or Maecuru group, having a thickness of 

 about thirty feet, consists entirely of coarse sandstones, and is 

 very fossiliferous. The only fossils of this section that have 

 been determined are the Brachiopods, which prove that the sec- 

 tion is closely related to the Upper Helderberg of North Amer- 

 ica, but has also many characters in common with the Ham- 

 ilton group. 



The second or Erere group has an estimated thickness of about 

 200 feet, is made up mostly of fine-grained micaceous sand- 

 stones, with some beds of black shale, and is underlaid by beds 

 of cherts. The fossils, which are mostly Brachiopods, are in 

 part identical with those of the Maecuru, in part, with those of 

 the Hamilton group of North America. The upper group, 

 called the Curua, is about 300 feet thick, and consists almost 

 entirely of black and yellowish shales, passing at times into 

 shaly sandstones. The only recognizable- fossils discovered 



