The Ohio Naturalist. 
[Vol. XI, No. 4, 
274 
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. ’ ’ 3 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 modem, of altered, 
but in general non-crystalline recks consisting of quartzites, 
metamorphic schists and crystalline limestones.” 4 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 unconformity 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.” 5 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 cliff, although so much decayed that one can 
thrust a knife into them anywhere. 
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. 
