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. 13 In 
Peru they consist of blue to gray and black clay slates, shales and 
graywackes, 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, 14 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. 15 The Silurian strata of the Bolivia-Brazil-Argen- 
tinc region are not very much folded but are faulted, tilted and 
often cut by intrusions of granite, porphyry, dioritc, 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. 16 Important veins carrying gold, 
silver, lead, tin, copper, zinc, nickel, etc., occur in the Silurian 
rocks and are 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 , Lingula ps, Tellinomya, Anodontopsis ). 17 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 unconform - 
ably, at one place on felsite and at another on syenite. 18 
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 Gcsell- 
schaft, Band XLVIII, 1896, pp. 743-772. 
16. Forbes, David, Loc. cit., p. 61. 
17. Clarke, J. M., The Paleozoic Faunas of Para, Brazil; Archivos do 
Museu Nacional do Rio de Janeiro, Vol. X, 1900, pp. 1-24. 
18. Derbv, O. A., Proc. Amer. Phil. Soc., Vol. XVIII, 1879, pp. 
167-169. 
