2 74 . The Ohio Naturalist. [Vol. XI, No. 4, 



No thicknesses are given and the value of the classification is 

 perhaps questionable, but it appears to be the most complete of 

 any thus far in print. 



Concerning the semi-crystalline schists and marbles (No. 4 of 

 the above section), Crosby says they "represent a horizon near, 

 but below, the boundary line between the Eozoic and Paleozoic." 

 And the granite (No. 1) "lies at the base of all the rocks of the 

 colony (British Guiana) and coarse veins of it have pierced all 

 the overlying formations including even the sandstone in one 

 place. ' '^ This rock is identified as Laurentian in age but it either 

 includes younger intrusives or is itself much younger, as the 

 sandstone said to have been cut by it is identified as Triassic. At 

 some places the granite is said to show a gneissic structure and 

 again to pass over into distinct gneiss. 



These old crystalline rocks are thought to have been above the 

 sea in earliest Paleozoic time, but that they have not remained 

 above throughout all the succeeding time is suggested by the 

 great mass of Triassic sandstones capping the hills over 10,000 

 square miles of British Guiana. 



The pre-Cambrian rocks of the highlands of eastern and 

 southern Brazil cover a much larger area and, as in the former 

 area, are in part covered by undetermined later formations. 

 They extend over more than 30° of latitude and 25° of longitude. 

 Here, as to the north, two great divisions of the rocks are repre- 

 sented. These consist of "two very distinct series, of which one, 

 the most ancient, consists of crystalline rocks, including gneiss, 

 gneiss-granite, and syenite, and the other more modern, of altered, 

 but in general non-crystalline rocks consisting of quartzites, 

 metamorphic schists and crystalline limestones."^ The section 

 is essentially the same as that given for the Guiana region and the 

 same great unconformity is recognized. The transitional rocks 

 above the unconfomiity are mainly quartzites and schists, with 

 some argillite, crystalline limestone and bedded iron ores. The 

 quartzite frequently passes over into ordinary sandstones, among 

 which is the well-known flexible sandstone — itacolumite. 



As an evidence of the age of the land-surface in this part of 

 Brazil, Branner says that "the fine-grained gneiss in the vicinity 

 of the city of Theophilo Ottoni, is so deeply weathered that one 

 seldom sees a hard rock face."^ The street and railroad cuts are 

 made in the decomposed rock. At one place near the railroad 

 station, the rock cut is 10 meters in depth and the schists stand as 

 a perpendicular clifi^, although so much decayed that one can 

 thrust a knife into them an^^vhere. 



3. Loc. cit., p. 493. 



4. Derby, O. A., Proc. Amer. Phil. Soc, 1879, pp. 155-178, 251-258. 



5. Proc. Wash. Acad. Sci., Vol. II, 1900, p. 187. 



