276 The Ohio Naturalist [Vol. XI, No. 4, 



over a region extending from northwest to southeast more than 

 700 miles and including an area of 80,000 to 100,000 square miles. 

 These rocks form the mountain chain of the highest Andes, rising 

 to a maximum elevation of 25,000 feet above sea-level.^''' In 

 Peru they consist of blue to gray and black clay slates, shales and 

 gra\^vackes, with a subordinate amount of sandstone. East of 

 La Paz the Silurian is thought to be fully developed and here 

 Forbes estimated its thickness at 15,000 feet,^"* but it is quite 

 probable that this includes also the Ordovician and a portion of 

 the Cambrian. 



Near Hanco in northwestern Argentine the Silurian is about 

 4,000 feet thick and consists of bluish gray to yellowish rough 

 uneven-bedded limestone interstratified with marl, and all quite 

 fossiliferous.^^ The Silurian strata of the Bolivia-Brazil-Argen- 

 tine region are not very much folded but are faulted, tilted and 

 often cut by intrusions of granite, porphyry, diorite, trap, etc., 

 and in the vicinity of these masses the strata are altered into 

 gneissic and schistose rocks whose sedimentary origin is only 

 occasionally to be recognized.''' Important veins carrying gold, 

 silver, lead, tin, copper, zinc, nickel, etc., occur in the Silurian 

 rocks and arc thought to have been formed prior to the extrusion 

 of the post-Paleozoic lavas. 



In the Lower Amazon region Silurian strata outcrop on the 

 Guiana side in a narrow strip (4 ± miles wide) along the southern 

 margin of the metamorphic rocks, from the Rio Trombetas 

 nearly to the Atlantic Ocean. On the river mentioned they have 

 been studied to some extent and a considerable fauna collected 

 (Orthis, Lingulops, Telliuomya, Anodontopsis).^'' Here they con- 

 sist of about 1,000 feet of hard argillaceous and fine-grained 

 micaceous sandstone, with some shale between the layers and 

 about twenty feet of schists at the bottom, resting unconfonn- 

 ably, at one place on felsite and at another on syenite. ^^ 



At the Morro do Cachorro the Silurian sandstones have a 

 grayish, yellowish or reddish color, are often banded, and dip to 

 the S. SW. at an angle of 5°. They frequently contain impres- 

 sions similar to those recognized in the Medina of North America 



13. Forbes, David, Ibid., p. 53. 



14. Forbes, David, Ibid., p. 61. 



15. Bordenberger, W., Leitschrift der deutschen geologischen Gesell- 

 schaft, Band XLVIII, 1896, pp. 743-772. 



16. Forbes, David, Loc. cit., p. 61. 



17. Clarke, J. M., The Paleozoic Faunas of Para, Brazil; Archives do 

 Museu Nacional do Rio de Janeiro, Vol. X, 1900. pp. 1-24. 



18. Derby, O. A., Proc. Amer. Phil. Soc, Vol. XVIII, 1S79, pp. 

 167-169. 



