ECONOMIC GEOLOGY
The exploration of western Brazil was stimulated by re
ports of gold. The first successes were in 1699 and in 1718,
when the discovery of the rich placers of Cuyaba led to the
opening of the remote interior. The Passagem Mine at
Ouro Preto, which was opened in 1817, occurs in gently
dipping pre-Palzozoic qQuartzites. The main lode is a sheet
of white quartz, which ranges up to 36 feet in thickness and
is streaked with layers of tourmaline, pyrrhotite, and arseno-
pyrite. Below it is a dark tourmaline rock up to 3 feet
in thickness, containing pyrite, pyrrhotite, arsenopyrite, and
quartz. According to Derby the Passagem Lode is a peg-
matite dyke intruded between the overlying Itabirite or
banded ironstone, and the underlying quartzite, and subse-
quently impregnated with gold and sulphide. The lode is,
however, probably a felspathic quartzite, in which pneuma-
tolytic solutions “altered the felspar into tourmaline and
deposited the metals (cf. O. A. Derby, Amer. Yourn. Sci. (4),
Xxxii, 1911, pp. 185-00; E. Hussak, Z. prakt. G., vi, 1898,
PP. 345-57).
IsoraTED GoLp-QuarTZ VEINS—SADDLE- AND LADDER-
Lopes — The earth-movements in some goldfields instead
of forming continuous fissures, produced isolated spaces,
which have been filled by short lodes. The Bendigo Gold-
field was discovered in 1851, and after its rich gravels were
worked out search was made for the lodes whence the gold
had come. Large quartz-“ blows” were found, and some
were so rich that they were broken up and crushed by hand
hammers. These bodies of quartz proved to be wedge-
shaped and they rapidly tapered out below. The view was
therefore held that as the field had no persistent quartz-lodes
like those of California the mines would be shallow. The
mines are in Ordovician slates which have been corrugated
by many parallel folds. As neighbouring quartz-blows
sloped in opposite directions, it was suggested that each
pair was the remains of a saddle or arch of quartz formed
over a fold. This view was established by the Geological
Survey of Victoria by E. J. Dunn, H. S. Whitelaw, and
H. Herman, and supported by Rickard. The latest report is
by H. Herman (Bull. G.S. Vict, No. 47, 1923), who has shown
that the field is a geosynclinal.
Saddle-shaped sheets of quartz were found one below the
