Quaternary Deposits, Etc .— Mills. 355 
slope, near the divide, at the head of a rapidly falling stream. 
It follows that the region has been upraised since the deposit 
of iron ore was laid down. 
One of the areas most productive of gold when the gold 
product of Brazil was largest was the one comprising the 
slope of Oaro Preto mountain facing southwesterly, south¬ 
erly, and southeasterly. The gold here was found principally 
in the cascalho at the bottom of a layer of iron oxides similar 
to the one which paves the plain between Inficionado and 
Agua Quente. These superficial iron ores are called in the 
country “ Canga.” The surface on that slope of Oaro Preto 
mountain is dotted over with pits sunk through the Canga 
for gold. The Canga is reported to be from 3-J- to 9 feet thick 
there. The surface on which it rests is a steep mountain slope, 
too steep to allow bog ore to be deposited in a broad sheet 
upon it. 
The gravels underlying and mingled with the Canga, as well 
as the gold found in them are the remains of a stratum or 
group of strata of thinly laminated, greenish talcose slates 
containing irregular masses of quartz and quartzite with 
arsenical pyrites, tourmaline and gold. It is overlaid by ita- 
byrite^which consists of laminae of pure, flaky specular iron-ore 
alternating with laminae of quartz and mica and iron oxides 
of rusty colors. On the slope this and other strata were de¬ 
composed, leaving quartz and gold and iron ores at the sur¬ 
face. At the foot of the slope the gold-bearing stratum or 
group of strata is found in place, at first softened, but grad¬ 
ually becoming firm and hard going down the dip which is in 
the same general direction (from southwesterly to south¬ 
easterly) as the slope. The softened portions of the gold- 
bearing slates have been excavated for gold, and at the Passa- 
gem mine the excavations have been carried down into the 
solid rock where it is overlaid by other strata. On the moun¬ 
tain slope the gravels would have been washed away if they 
were not protected by the Canga, and during the long process 
of decomposition of the rock from which the gravel and gold 
originated, before the Canga was deposited upon them, the 
slope must have been much less steep than now. Moreover, 
the Canga, whether deposited in a bog or by springs on a 
slope, was soft when first precipitated, and could not have re¬ 
mained widespread over so steep a slope. 
