158 GEOLOGY AND MINERALOGY, 
i , 
their geological constitution is similar to that of California ; and 
it is also on the eastern sides of the range that these metals 
have been plentifully obtained. Sufficient historical evidence of 
the presence of gold upon the Isthmus, or in its immediate prox- 
imity, has been produced ; and there is little doubt if the sands 
of the Coatzacoalcos, Uspanapa, and other rivers on the east side 
were examined, the presence of gold would be detected. 
The Andes of South America are the great storehouses of sil- 
ver for the world. The richness of the mines is almost incon- 
ceivable ; but their great elevation above the sea makes the cli- 
mate so cold and the labor of mining so great, as not to be a 
profitable speculation, except in a few cases. 
In Mexico the ores are equally rich, but at a comparatively 
low elevation, so that where the ore is found in quantity, it is 
always advantageous to work it. Its position is various. The 
silver mines of Comarga occur in sienite. In Guanaxuato; the 
richest mine in Mexico, the ore occurs in talcose slate. At Real 
del Cardonal-Xacala, in Limo del Toro, it is found in transition 
limestone. The average richness is four ounces in each quintal 
of 102 pounds. 
The general position of silver ores is in veins which traverse 
the primary and older of the secondary stratified rocks ; but 
especially the former, as well as in the unstratified rocks, the 
granites, and porphyries which accompany the above. In the 
limestone the silver is generally associated with lead ore, which 
is then termed argentiferous galena. The proportion of silver 
found in English lead mines is twelve ounces to the ton of 2240 
pounds ; but it is profitable to separate it when it is as low as 
eight ounces, or one grain in half a pound of lead ore. Silver is 
occasionally found in Northern Mexico, mixed with iron ore and 
ochre chiefly, as chloride of silver, or horn silver. The same 
system of rocks which are metalliferous in the other districts 
of Mexico exist upon the Isthmus ; and the limestone contains 
galena which is argentiferous. An exploration of the metamor- 
phic rocks might lead to the discovery of some valuable veins. 
Iron ore has been found in quantity in three places apart from 
eacb other. One locality is in the hills north of San Juan Gui- 
chicovi. The second is among the shale which crop out near 
