OOo 
Correspondence. 127 
In Rio Grande do Sul the channels of the present drainage have 
been cut down through the loess and cascalho, and the country has 
evidently been raised since the loess was deposited. The same eleva- 
tion undoubtedly caused the terracing along the coast. 
I had occasion to examine very carefully the gold bearing gravels 
at the Lagoa de Maga in the province ‘of Rio Grande do Sul. The 
lake is simply a deepened and widened section of the Comaquam river 
near the village of Lavras in latitude, about 10’, 49’ west of Rio de 
Janeiro, and longitude about go’, 44’ south. The lake is said to be 
about 700 feet above sea level and occupies a basin eroded from the 
hard porphyry to a depth of 7.5 feet at the lowest below the rocky 
outlet. The hard feldspathic porphyry is softened in places in the 
bed of the lake, and excavations in the lake show softening to a depth 
of at least 12 or 15 feet, but the softening is by no means as exten- 
sive in this part of Rio Grande do Sul as it is in the Province of 
Minas. It does not extend over all of the bed of the lake. 
Resting on the rock is a thin, irregular, varying sheet of gravel 
and sand, which is often entirely wanting; where it exists it is but a 
few inches thick. It consists largely of rounded, smooth, quartz peb- 
bles and also of more irregular pieces of porphyry. The quartz pebbles 
are evidently from quartz veins which traverse the porphyry in the 
immediate neighborhood. 
This is the cascalho, or gravel, in which the gold is found. It is 
exposed on the bed-rock above the level of the lake, and it is proba- 
bly distributed in patches over the surface of the rock in the neighbor- 
hood generally: but nowhere within my observations was there found 
a continuous sheet over any extended area. I did not see it more than 
6.6 feet thick, except in some narrow furrows or channels in the rock 
in the bed of the Lagoa where the gravel and local large boulders 
alone formed a mass three feet thick. 
Next above the gravel and resting directly on the rock where the 
Cascalho is wanting is the loess-like deposit, which is spread over the 
surface of the country generally. It consists of a fine clayey material 
of yellow or drab color where not darkened by organic matter and of 
uniform texture. However, streaks of sand appear in the lower part of 
it, and sometimes it passes downward into a sandy layer which may 
have been deposited at the same time as the gravel, but I found no gold 
in it, though I tested it repeatedly by washing. Near the surface the 
loess becomes mingled with organic matter, and passes upward into 
a dark soil. In one place an excavation near the lake showed a thick- 
ness of 13.6 feet of loess. 
Next foilowing the loess are irregular deposits of sand found only 
along water courses, but often at higher levels than the water now 
attains. I did not see these sands anywhere lying on the loess. 
The lake crosses the outcrop of some veins of quartz, and the grave! 
is mostly limited to the immediate neighborhood of such outcrops, ex- 
cept at the bend of the still water of the pool. 
James E. Mis. 
Quincy, Plumas county, California, July 20, 1888. 
