1S79.1 



249 



itacoluuiite and other quartzites. And back of this again at the west bor- 

 der of the famous Campos Geraes grass plain, the Serra de Esparanga, also 

 about 3000 feet high, composed of Devonian (and carboniferous?) fossilif- 

 erous soft red sandstones resting on the shales and sandstones of the great 

 plain, and having a bold escarpment towards the east, like our Alle- 

 gheny-Cumberland backbone range. The upper part of the escarpment, 

 however, is an outcrop of amygdaloidal and porphyrinic trap 350 feet thick, 

 and full of agates, which forms the long back or west slope perhaps all the 

 way to the Parana river, the border of Bolivia ; and this is conjectured by 

 Mr. Derby to be of Trias age. 



2. The trend of the formations resembles that of the Atlantic border 

 of the United States, being from west of south to east of north. But while 

 the general geographical order is the same, namely, — Azoic, on the east 

 along the coast, and Devonian on the west, — there are striking differ- 

 ences, first in its great simplicity, and secondly, in the Trias and trap 

 lying west of the Devonian. All three ranges have escarpments towards 

 the east. A very high (2000 feet) plateau fills in the space between the 

 first and second ranges ; and another plain sloping gently westward, and 

 1500 to 2000 feet above the sea, fills in the belt 100 miles wide between the 

 second and third ranges. There is, therefore, a general uptilt of this part of 

 Brazil towards the east ; higher and higher rocks coming in as one goes- 

 west, and the whole slowly settling into the great central plain of South 

 America, as ours do under the plain of the Mississippi Valley. 



3. The drainage system has some striking features of resemblance to that 

 of the United States when we consider the short rivers which flow east- 

 ward into the Atlantic, and the long rivers, like the Upper Ohio, Kenawha 

 and Tennessee, which flow through the Allegheny Mountains down, dip, 

 westward into the Mississippi. For Mr. Derby describes four main rivers : 

 1. The short Ribeira which alone flows east, through the granite range, into 

 the Atlantic ; 2. The long Iguassu on the south, and 3, the long Parahapa- 

 mena on the north, both of which drain the first high plateau and flow in 

 opposite directions from one another, and then turn and cut westward into 

 the face of and through both escarpments, and through the second plain ; 

 and 4, the long Ivahy, between them, which cuts across the second plain 

 and third escarpment westward, also into the Parana. 



4. Mr. Derby shows that the pot holes of the Tibagy (a branch of the 

 Paranapamena) got their diamonds and other crystals not directly from any 

 older formation than the Devonian, for the Tibagy drains nothing but 

 Devonian country. But again he shows that the diamonds, &c, must have 

 been set free by the erosion of the Devonian sandstones as pebbles or sand - 

 grains or fossils ; for the Devonian sandrocks are not in the least metamor- 

 phosed. The diamonds must therefore have been originally derived from 

 much older itacoluuiite rocks, &c, out of which the Devonian rocks them- 

 selves were constructed by erosion and deposition. 



Dr. LeConte, at the request of Mr. Dubois of the IT. S. 

 Mint, exhibited a very fine specimen of laminated native 



PROC. AMER. PHILOS. SOC. XVIII. 103. 2f. PRINTED JUNE 10, 1879. 



