3o8 TJie Aincrican Geologist. November, i899 
have come up along the fault planes. Near the surface the ores are 
in the form of oxides, sulphates and carbonates with some sulphides. 
Downward these soon give place to the pure sulphides. Silver- 
bearing galena is the most important ore, blende, pyrite, polybasite, 
tetrahedrite and tennantite also are abundant. Small quantities of 
arsenic, cadmium, cobalt and nickel are present. The paragenesis of 
the vein materials is of special significance. 
The deposition of ore is found to be coextensive with the faulted 
district. Where the fractures fade out into unbroken strata the ores 
disappear. The vertical extent of the ore bodies is unknown. Present 
mining operations give a vertical range of over 3,500 feet between 
points where the ore is continuous. 
The chapter on glacial action in the region presents some features 
that deserve more than passing mention. "The evidence goes to 
show that at a relatively remote period the Aspen district was covered 
by a great ice sheet, which moved west, away from the Sawatch 
range, over hill and valley. This glacier carved the surface into typical 
glaciated, rounded and drumlinoid forms, and excavated the softer 
shales and sandstones more than the resistant granite. The debris of 
this glacier is found on top of the highest mountains of the district. 
Subsequently this ice sheet shrank into separate glaciers, which fol- 
lowed the valleys of pre-existing streams and in large measure carved 
them into their present forms. These valley glaciers, by erosion along 
their sides, caused a steepening of the mountain slopes, and so brought 
about the removal of most of the previously accumulated drift. It 
thus happens that at present the sides of the mountains often appear 
bare and unglaciated, and the drift of the earlier glaciation is found 
only on the summit. At a still later stage in the glaciation the valley 
of Roaring Fork, was occupied by a long, narrow glacial lake, which 
probably resulted from some temporary dam. During most of its 
existence the surface of this lake was 400 or 500 feet above the present 
town of Aspen, and into it the dying glaciers of Hunter creek and its 
water emptied." An appendix on the Measurement of Faults contains 
some distinctions to be recognized with advantage. Altogether the 
geology of the Aspen district is a notable contribution to our knowl- 
edge of the ores and geology of Colorado. It is clear, terse, and easy 
to understand. If more of our accounts of important mining districts 
were modeled on the same plan our knowledge of ore deposits in gen- 
eral would be in a far more satisfactory condition. c. r. k. 
The Crystal Falls Iron-bearing District of Michigan by J. M. Cle- 
ments and H. L. Smyth; with a chapter on the Sturgeon River Tongue, 
by W. S. 'Qwt.'EY, ivith an Introduction hy C. R. Van Hise. (U.S. 
Geol. Survey, 19th Ann. Rept., pt. 3, pp. 1-151, pis. i-ii, 1899.) 
The Crystal Falls district is in the northern peninsula of Michigan, 
immediately adjacent to Wisconsin. This district, as far as its present 
development is concerned, is not of the economic importance of either 
the Marquette iron-bearing district to the northeast nor the Me- 
