96 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
that has generally been assumed to be Archean is the prevalence of 
distinctly sedimentary beds, largely quartzites of pre-Cambrian age, 
which form the crest of the ridges in either group of hills. Around 
the lower flanks of the ranges, on the other hand, and, so far as can 
be seen, in the intermediate Platte Valley, the basement rocks are 
granite and gneisses like those called Archean in Colorado, a^d the 
sedimentary rocks are seldom, if ever, seen. 
The sedimentary series in the Encampment Range are quartzites, 
with some limestones and conglomerates, penetrated by sheets and 
dikes of eruptive rock, mainly diorite, and closely compressed into 
folds with an east-west strike, the whole highly metamorphosed. 
That the same series of rocks occurs in the Medicine Bow Range is 
evident from observation, though the structure could not be made 
out with any definite ness in so short a visit, owing to the covering of 
forest and wash on the plateau. According to the Fortieth Parallel 
reports, Medicine Peak is a mass of white quartzite with a general 
synclinal structure. The same rock is said to occur on Douglas 
Creek a few miles below the Rambler mine, and quartzites were 
observed in considerable masses on the ridge between French and 
Mullan creeks a few miles west of that mine. At the Rambler mine 
itself and for a considerable distance on either side, as shown by the 
dumps of the different prospects, the rock is diorite. That at the 
Rambler mine is coarsely granular, with thoroughly granitic struc- 
ture and some development of pegmatitic phases, and is thought to 
be part of a considerable stock breaking through quartzites and 
underlying gneisses. 
Microscopical study of two specimens from the bottom of the shaft, 
by Mr. Waldemar Lindgren, shows the latter rock to be a hornblende- 
biotite-diorite of normal type, its feldspars being labradorite, with 
quartz occurring sparingly in grains between the feldspars, and mag- 
netite in small contact grains, mostly embedded in hornblende or 
biotite. As secondary minerals are found sericite and epidote in the 
feldspars, and a little chlorite in places in hornblende and biotite. 
Some red hematite occurs in seams, and p3 7 rite in grains and cubical 
crystals, generally in the hornblende, and always associated with a 
little chlorite. 
RAMBLER MINE. 
The mine was originally opened by an incline, but after the discov- 
ery of the large bodies of ore a vertical two-compartment shaft was 
sunk, through which access is now had to the workings. At time of 
visit the mine below the 100-foot level was inaccessible. Above this 
level and for some little distance below, both ore and country rock 
are much altered, and so much transmigration of the former has 
taken place that it was quite impossible to determine the original 
form and nature of the deposit. 
