208 
LACCOLITHIC GENESIS 
quently a prime factor. Since De Launay ® first called attention 
to the variation of metalliferous veins with depth, the theme has 
received wide notice. The author’s main thought, and the one 
which is particularly suggestive, is that owing to enormous eros¬ 
ion which many parts of the earth’s crust have undergone we 
are able to observe in many instances portions of veins which 
are actually formed at great depths. Discussing the French 
author’s conclusions, Krusch ® cites a number of localities in 
which in relatively late gological times great denundation has 
taken place, the thickness of the rock-prism removed amounting 
to three to five miles or more. Vogt also supports the opinions 
of the writers just mentioned, but is inclined to go somewhat 
farther by greatly increasing the denundation units, by miltiply- 
ing the ordinary figures by ten. Van Hise although leaning 
towards the extreme view mentions typical examples which 
plainly do not support his contention, but demonstrate that the 
depths at which these particular contact-ore bodies appear fall 
very short of the zone of rock-flowage. Data recently presented 
on the Tuertos Mountains by Yung and McCaffery^^ furnish 
critical evidence upon the moot points concerning the distance 
from the surface at which the ore deposits were originally formed. 
Some of these results are personally substantiated; and others 
are added from the neighboring field of the Ortiz. 
Taking into account the several unconformity planes in the 
local rock-section as representing great erosion intervals it is 
indicated that when the laccolithic masses were formed the 
thickness of the superincumbent strata could not have possibly 
been more than 3000 feet. In the instance of the Ortiz intrusion 
a thickness of 2000 feet of sediments would in all likelihood be 
nearer the truth for the actual volume of strata floated. These 
figures seem also to obtain for the neighboring Tuertos mass. 
The Maryville laccolith, or batholith, of Montana is believed by 
Barrell to have reached a level within 4000 feet of the surface. 
8 Rev. gen. des Sci. purSe et appliques,- t. XI, 1900. 
9 Zeitsch. f. prakt. Geologie, p. 317, 1900. 
10 Trans. American Inst. Min. Eng., Vol. XXXI, p. 158, 1901. 
11 Mon. U. S. Geol. Surv., Vol. XEVII, p. 1056, 1904. 
12 Trans. American Inst. Min. Eng., Vol. XXXIII, p. 355, 1903. 
13 Economic Geology, Vol. VI, p. 365, 1909. 
14 Prof. Pap. U. S. Geol. Surv., No. 57, p. 81, 1907. 
