Review of Recent Geological Literature. 307 
Geology of the Aspen Milting District, Colorado. By Josiah Ed- 
ward Spurr. (U. S. Geo]. Sur. Mon. XXXI, xxxiii and 260 pp., 43 pis., 
and atlas. Washington, 1898). 
The Aspen mining district presents, in addition to its great eco- 
nomic importance, an interest that is unusual from a purely scientific 
standpoint. The scope of the report is summed up as follows: Chap- 
ter I treats of the geological formations both sedimentary and igneous, 
their original structure and the conditions under which they were 
laid down or intruded; Chapter II considers the physical changes 
which have come about since their deposition and consolidation, con- 
sisting mainly of folding and faulting; Chapter III gives full descrip- 
tions of the mines and the productive localities; Chapter IV discusses 
rather fully the chemical changes which came about subsequent to 
or were attendant upon the physical changes, and were produced 
chiefly by metasomatic change, dolomization, silicification, ore depo- 
sition, and other phenomena; Chapter V is a short sketch of some 
of the surface changes which have occurred in comparatively recent 
times, since ore deposition. An introductory chapter is by S. F. 
Emmons. 
The general geological features are thus tersely stated: "The fun- 
damental rock in the Aspen district is a granite, with occasional 
gneissic structure. Above this come successively the sedimentary beds 
of the Cambrian, Silurian, Devonian, Carboniferous, Juratrias and 
Cretaceous. The beds of the Cambrian, Silurian and Devonian are 
comparatively thin, while the Carboniferous, which is divided into 
three distinct formations -the Leadville, the Weber and the Maroon- 
attains a great thickness. The Juratrias and the Cretaceous are also 
very thick, the latter containing the various subdivisions of the Da- 
kota, the Colorado, the Montana and the L-aramie. Separating these 
different beds at intervals are various unconformities and planes of 
erosion, which help one to read the history of the rock and to under- 
stand the conditions under which the beds were laid down." 
In Cretaceous time probably were intruded the igneous masses. 
After this came dynamic disturbances giving rise to the principal 
mountains of the district. At first profound faulting occurred. From 
that time to the present day faulting has gone on. The result has 
been to produce a remarkable series of fractures, many crossing one 
another. It is noteworthy that the slipping is still going on in a 
measureable degree. Benclj-marks of the engineers have moved sev- 
eral feet in the course of a decade. The timbered mine entries origin- 
ally nearly rectangular in shape, are. in a short space of one or two 
years, made very decidedly rhomboidal in cross-sections — so much 
so that the car tracks have to be laid on short ties which are from 
time to time moved laterally until finally they abut the side of the 
entry. The upper portion of many shafts has moved entirely across 
the lower portion, in some cases shutting off the latter completely; in 
others the two ends arc connected by a short incline. 
The discussion of the ores and their deposition are of exceptional 
interest. They are regarded as precipitated from heated waters that 
