NOTES ON THE ARGENTINE REPUBLIC. 
11 
its mass being filled by sulphate of lime, quartz, and hydrated 
oxide of iron. 
The crystalline sulphate of lime, native sulphur, and amor- 
phous silica, which are found in so many places, suggest that 
highly heated vapours have played an important part in the 
history of these deposits. 
How the cavities were first formed is not apparent, but the 
suggestion of some geologists that acidulated water may have 
caused them, receives some colour from the mode of occurrence 
of granite in varying places. In fig. 2, Plate I., the section 
of a pipe-vein is shown with the granite forming the lower 
wall. The surface of the granite, from which the pyrites and 
oxide of iron can, in many places, be easily separated, looks 
rounded and slightly decomposed, but the pyrites is compact 
and firm. 
The granite in several places cuts the limestone nearly at 
right angles, and where this has been the case, large deposits 
of pyrites and gossan occur. The active agent in the decom- 
position of pyrites has been moisture, and this action is still 
going on, as witnessed in pyrites found exposed in the workings. 
Some of this, where moisture percolating from above has 
reached it, has crumbled and become oxidized, the action 
spreading from small nuclei to the mass around. 
The Spaniards and their descendants, in working these pipe- 
veins, both atGrualilan and at Gruachi, twenty miles further north, 
in a continuation of the same formation, only removed the 
gossans, which would constitute from one-half to two-thirds of 
the mass, and left the solid pyrites for future workers, who 
should have the means of separating the sulphur. They ex- 
tracted the gold and silver from, these gossans by crushing 
the ore between stones, many of which are still found in the 
districts. These were of the form shown in fig. 3, Plate I. 
The ore selected by them, a quartzose gossan, was broken down 
to the size of Barcelona nuts, and placed with a little water on the 
lower block, a piece of limestone about three feet long and two 
feet wide, with the centre cut out, so as to leave sides to guide 
the upper block, or granite muller. Then two men moved 
the upper block slowly backwards and forwards over the lower 
one, until the ore became reduced to a powder as fine as flour ; 
a small quantity of mercury was then added, and the grinding 
again continued until the mercury was supposed to have amal- 
gamated with all the metals. The ground ore and mercury 
were then removed to shallow wooden pans, and washed by a 
free use of water ; the mercury was again used in this way 
until it became saturated with amalgam, when the excess of 
mercury was removed by wringing in a soft piece of skin. 
The balls of amalgam thus obtained were covered with clay, and 
