BMMons.] INVESTIGATION OF METALLIFEROUS ORES. 21 
(3) Ore Deposits of the Rico Mountains. Colorado, by F. L. Ransome. 
This urea is also situated in the San Juan Mountains, and the eco- 
nomic work followed an areal survey by Mr. Cross and his party, but 
the conditions there differ from those in the Silverton quadrangle, in 
that the ore deposits are concentrated within a limited area. It could 
thus be properly made the object of a special survey, especially as the 
peculiar nature of the ore deposits renders it unusually worthy of such 
detailed study. Unfortunately, by the time the Survey was in condi- 
tion to undertake this examination the principal mines had been prac- 
tically worked out and a large proportion of their underground work- 
ings had become inaccessible. This fact has given rise to some unfa- 
vorable criticism of the methods of the Survey on the part of those 
who consider that the most important result of its work is the imme- 
diate aid afforded by it to the miner in the development of the mines 
of the particular district under survey. In this case geological-con- 
ditions are such that it would have been impossible to make a satis- 
factory study of the ore deposits until the peculiarly complicated 
geology of the whole quadrangle had been worked out. Further- 
more, few mining districts present conditions so favorable for a definte 
determination of some of the undertying principles controlling ore 
deposition, conditions which the able mining engineers who had at 
different periods had charge of the principal mines of the district had 
of necessity been unable to completely understand, because the true 
geological relations of the deposits were not yet known. 
Mr. Ran some's report, in spite of the obstacles he had to contend 
with in making the examination, gives a remarkably able and satis- 
factory delineation of the conditions governing ore deposition along 
bedding planes of limestone under impervious shales, in part replacing 
a bed of gypsum, and associated with well-defined fissure systems and 
different ial movements along bedding planes, and of the genetic con- 
nection of its deposition with the intrusions of igneous rock in the 
immediate vicinity. It constitutes a most valuable contribution of 
well-determined facts bearing upon the general theoiy of ore deposits. 
(4) Geology and Ore Deposits of the Elkhorn Mining District, Montana, by 
W. H. Weed and Joseph Barrell. 
This is mainly the study of one great mine; a mine, moreover, that 
is practically worked out, and to which are applicable the same criti- 
cisms that were made of the Rico report. Here, also, the study has 
been extremely fruitful in presenting facts bearing upon the forma- 
tion of a rather unusual type of ore body. The deposit is considered 
Iby Mr. Weed to be in the nature of a saddle-reef deposit, formed in 
crushed limestone under a shaly roof within arches of pitching anti- 
clines, lie further considers the ore to have been deposited by hot 
solutions rising from a cooling batholith of eruptive rock. There is 
also evidence of secondary sulphide enrichment in the ore. 
