Bahia
region by Prof. Orville A. Derby, who was engaged by the State of Bahia
to study the geological occurrence of the carbonados. The article in
question is his report of this work, and has been translated into
English by Prof. John C. Brainier for publication in this country."
The
region examined was the basin of the Paraguacu River, the tipper part
of which contains the Chapada Diamantina, or diamond plateau of Bahia.
The river traverses four regions or belts of entirely distinct
geological character. The first of these takes in all the headwaters of
the Paraguacu and its main affluent. the Santo Antonio: the second
reaches from the Falls of Passageni do Andarahy. some 50 miles, to
Bededouro; the third extends about 200 miles, down to Maragopipe. and
the fourth is a narrow belt adjacent to the coast.
The diamonds and carbons are characteristic of the first region, and occur occasionally in the second and third. At all
the localities examined by Professor Derby they appear in connection
with a thick bed of conglomerate about the middle of the hard sandstone
formation. In many places, indeed, he states that the gravels worked
for diamonds are simply this same conglomerate decomposed in place, and
not a more recent superficial deposit. This fact has already been
recognized in the State of Minns Geraes, where also, at Grao Mogol.
diamonds have been taken from the hard conglomerate itself. It is clear
that in Brazil the conditions of diamond occurrence bear no resemblance
at all to those in Africa. All the indications point to the
conglomerate as the source of the diamonds, and the recent
unconsolidated gravels are richest in the vicinity of outcrops of this rock, which miners call pedra cravada.
If
this determination of the conglomerate as the home of the diamonds be
correct. Professor Derby remarks, the supply must be enormous, and all
that has been done heretofore is trivial in comparison. Only a small
part, however, of the deposits can be worked with the methods in use
thus far. Whether modern scientific processes, using the hydraulic
power so abundant in the region, can operate the beds at an actual
profit remains a question for future solution.
Elsewhere than in the vicinity of the Serra das Lavras and its conglomerate there are few diamond occurrences, but still
some are known that have interest. Along the lied of the Paraguacu. in
particular, there are various points where diamonds are obtained by
diving, and some of these are so far from the Serra that it is not easy
to see how the source can
be so distant. The principal locality is at the falls of Funil. near
Bebedouro, on the eastern edge of the second region or zone mentioned
above. Here the fall is formed by a heavy lied of conglomerate
resembling the one in the first zone, but almost certainly of later
age. It rests upon granitic rocks and its pebbles are largely thence
derived, so that the " formaoao." or diamond gravel, is quite different
from that of the first zone. The sandstone of the second zone, however,
must be largely derived from the first, and the contained diamonds may
be thus accounted for.
These
accounts should be compared with that by Mr. H. W. Furniss, dealing
with the same region and reviewed in the report of this Bureau for
1002.6
There
is another diamond region in Bahia—that of Cannavieras and the valley
of the 1'ardo River, and the southern part of the State/ This,
Professor Derby remarks, is the only diamond occurrence in Brazil at
all near the sea. It is a region of but slight elevation, only about
100 meters, largely wooded, and with a thick soil that obscures the
rocks. At points, however, in the val-
a) Econ. Owl., vol. 1. No. 2. Nov.-Dec, 1905. pp. 1S4-142.
b) "Mineral Resources C. S. for 1902, U. S. Geol. Survev, 1904, pp. 816-822.
c) Ibid, p. 810.
