1883. ] The Naturalist Brazilian Expedition. 1013 
ymo, and farther on in the bed of the Arroio da Porteirinha. By 
the decomposition of this and of porphyritic rocks farther up the 
river, immense quantities of agates, chalcedonies and cornelians 
have been formed ; very fine specimens may be picked up in the 
streets of São Jeronymo, and they are found all along the river 
shore to the mouth of the Arroio dos Ratos. Similar and still 
finer agates are found to the north and west of this point, some 
of them being nearly a meter in diameter ; some of the finest are 
exported to Germany, and the trade is already of considerable 
importance. The price paid for agates in Porto Alegre is ten 
milreis, or about $4.40 per barrel. 
The river shore at Sao Jeronymo and opposite Triumpho is 
formed of granite, this being the last that I have seen of the rock 
in this direction. The stratified beds farther north overlie the 
coal rocks. I shall have occasion to speak of them in another 
article. 
In the bed of the Arroio da Porteirinha, and on the campos 
between it and the coal mine fragments of silicious wood are 
found ; logs of this are so well preserved that they might readily 
be mistaken for half-rotten posts. Similar silicified wood occurs 
in large quantities in the districts to the north of the Jacuhy, 
but I have never found it in place. It is clearly much newer than 
the coal rocks, probably Quaternary or recent. 
Passing in review the geological section which I have de- 
scribed, it will be seen that the basal granite forms a series of 
cast-and-west ridges, successively increasing in height from the 
Jacuhy to the Serra do Herval. The Serra, so far as I have any 
knowledge of it, is formed throughout of granite, and on the 
pposite or southern side there are other granite ridges, succes- 
sively lower, to the Rio Camaquam ; the coal rocks, if I ma 
trust to the information of herdsmen, reappear in valleys between 
these. The Serra do Herval must therefore be considered as the 
backbone of an area of upheaval which raised not only the gran- 
ite but the overlying gneiss and coal rocks. The dip and strike 
are modified by intrusions of trap, but the general inclination is 
Ways aavay from thé central ridge. Thus, in passing north or 
South from the Serra do Herval we find successively newer rocks 
On the surface, and this rule holds good for some distance beyond 
the Jacuhy. 
Unfortunately the age of the coal beds is still somewhat prob- 
“ematical, Owing to the absence of fossils; the only recognizable 
