Monograph XII. is devoted to the geology and mining industry
of Leadville, and is from the pen of S. F. Emmons. The work
was begun under King's administration and completed and pub-
lished in 1886 under PoweH's. A preliminary abstract appears in
the Second Annual Report of the Director, 1882. The work,

* The Quarterly, vol. x., pp. 122 and 329.
REVIEW OF LITERATURE ON ORE-DEPOSITS. 229

which consists of one large volume of 770 pages and 45 plates,
and an atlas of 35 sheets, opens with a brief history of Leadville
and an account of its general geography. The general geological
structure of Colorado follows and a detailed description of the
Mosquito Range, lying just east of Leadville itself. The moun-
tains consist of Archaean gneisses and schists on which were
deposited up to the close of the Cretaceous io,ocx) to 12.000 feet
of conformable sedimentary beds. About the close of the Creta-
ceous a great intrusion of eruptive rocks into these sedimentary
strata took place, piercing them as dikes and penetrating between
the beds in enormous sheets. The mineral solutions bringing the
ores followed the eruptive rocks and formed the deposits chiefly
along the under contact of the sheets and the upper sides of beds
of blue dolomitic limestone belonging to the Lower Carboniferous.
The elevation and faulting of all followed in the general up-
heaval of the Rocky Mountains at the close of the Cretaceous or
about the beginning of Tertiary times. The geology of Leadville
itself is then described and the numerous faults carefully traced.
