VADOSE ORE DEPOSITION 
287 
come the first quantitative data on the magmatic derivation of ore- 
materials. Laccoliths present about the only definite features 
which we know of concerning the expulsion of metallic vapors 
from cooling magmas. In those laccolithic masses which have 
come under special observation certain relationships are establish- 
able between the metallic content of the cooled magma and the 
vein-phenomena displayed around the periphery. Ore-veins ap¬ 
pear to form a surprisingly small proportion of the entire metallic 
content. In comparison, the part permanently retained as acces¬ 
sory components by the rock-mass seems to be large. The total 
volume of metal lost through expulsion as soluble compounds in 
the form of vapors is at best difficult to accurately estimate; but 
there is as yet no evidence to show that it is nearly so large as has 
been sometimes regarded. 
In illustration, the granites and rhyolites of southeastern Mis¬ 
souri show no indications of the presence of metallic elements, yet 
Robertson’s exact analyses of perfectly fresh rock-samples 
demonstrate that large quantities of several metals are locked up 
in these masses. Calculated for a square mile of area and 500 
feet in thickness the granites contain about 45,000 tons of galena, 
65,000 tons of zinc-blende, and 135,000 tons of chalcopyrite; and 
the same volumes of the contiguous rhyolites, or porphyries, con¬ 
tain 75,000 tons of galena, 45,000 tons of zinc blende, and 60,000 
tons of chalcopyrite. 
At the southern end of the Rocky mountains, in central New 
Mexico, there is a remarkable group of four lofty laccolithic piles 
known collectively from the earliest times as the Gold Mountains. 
Their names are San Ysidro, Tuertos, Ortiz, and Los Cerrillos. 
On the Tuertos Group is situated the famous San Pedro Copper 
Mine. The areal exposure of the grano-dioritic porphyry of 
which it is composed is about five square miles. The country- 
rock itself assays about $1.00 a ton,^® and the mass thus contains 
gold equivalent to the present annual production of the entire 
United States. Although the first lode-mining in this country was 
carried on in the neighboring Ortiz district workable veins are 
exceptionally rare. At the San Pedro Copper Mines, the gold 
seems to be carried chiefly in the chalcopyrite. 
27 Missouri Geological Survey, Vol. VII, p. 479, 1894. 
28 Trans. American Inst. Mining Eng,, Vol. XEI, p. 148, 1910. 
