G, ATTWOOD ON SOUTH-AMERICAN GEOLOGY. 587 
The iron which exists in rocks contains more oxygen when the 
rocks have been exposed to the atmosphere, or when they occur near 
the surface, than when the rocks are found some depth below the 
surface and sheltered from the weather. The unaltered rock con- 
tained 12:34 per cent. of protoxide of iron and no peroxide, whilst 
the highly weathered contained no protoxide but all peroxide. 
Lime shows itself to be readily soluble, the unaltered rock haying 
contained 9°65 per cent. of lime and the highly weathered only 2:37 
per cent. 
Magnesia also is proved to be readily carried away by the wea- 
thering, but not in the same ratio as the lime. 
Potash and soda are very sensitive to weathering, but soda much 
more so than potash. 
The quantity of combined as well as the hygroscopic water in the 
unaltered and the weathered and highly weathered portions proves 
that rocks contain a larger quantity of water both combined and un- 
combined when they are exposed to the atmosphere than when they 
are sheltered from its influences. 
The diabase rock occupies a large area, and in it are found the 
gold-producing veins of the Caratal mining-district. The veins are 
very quartzose and numerous, and all contain gold; but, owing to 
the present high rate of wages and the great cost of procuring sup- 
plies from the Orinoco river, only eight or nine of the richest are 
now being worked. 
The Chile vein is one of the principal ones; and it has been traced 
about one mile in length. It has a dip of 55° to the south, and an 
E. and W. strike ; it averages in thickness about 4 feet, and is a true 
fissure vein. The yield of gold at 200 feet in depth gave between 
five and six ounces of standard gold to the ton (2000 lb.), whilst 
near the surface it only turned out about one ounce per ton. A 
large quantity of gold was extracted in the form of nuggets and 
small grains by washing in wooden bowls (bateas) the alluvial soil 
in the vicinity of the outerops. The present yield of gold from the 
Caratal and Las Pastora mining-districts is about 130 ,000 ounces 
per annum ; but it is rapidly increasing as the country is becoming 
more populated. 
About 30 miles due west from Potosi a new district has lately been 
opened ; large quantities of gold nuggets have been found close to 
the surface in the alluvial soil, and gold-bearing veins have been 
discovered near the washings. The rock is diabase and similar to 
that of the Caratal district. 
Conclusion.—lt is a remarkable fact that although quartz veins 
are found in great numbers all the way from the port of Las Tablas to 
the Caratal mining-district, yet none of them have so far been 
found to contain the precious metals, z. ¢. in any appreciable quantity, 
until the diabase is met with. 
All the rocks analyzed show a higher percentage of silica than is 
generally found. 
The independent examination made by Prof. Bonney, from micro~ 
scopical sections alone, as to the character of the rocks agrees closely 
