OUTLINE OF THIS MONOGRAPH. 
The various divisions in the geology of the Aspen district are treated in this report in 
chronological order. Thus. Chapter I treats of the original structure of the sedimentary and 
igneous rocks and of the conditions under which they were laid down or intruded; Chapter IL 
treats of the physical changes which have come about since their deposition, consisting mainly 
of folding and faulting; Chapter IV treats of the chemical changes which came about subsequent 
to or were attendant upon the physical changes, and were produced chiefly by metasomatic 
interchange, dolomization, silicification, ore deposition, and other phenomena; and Chapter V isa 
slight sketch of some of the surface changes which have occurred in comparatively recent times, 
since the ore deposition. Between the description of the physical and chemical changes is inserted 
a chapter describing in detail the mines and productive localities, this description being essential 
to the understanding of the various geological phenomena, especially that of ore deposition. 
The fundamental rock in the Aspen district is a granite, with occasional gneissic structure. 
Above this come successively the sedimentary beds of the Cambrian, Silurian, Devonian, Carbon- 
iferous, 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 Creta- 
ceous are also very thick, the latter containing the various subdivisions of the Dakota, the Colorado, 
the Montana, and the Laramie. Separating these different beds at intervals are various uncon- 
formities and planes of erosion, which help one to read the history of the rock and to understand 
the conditions under which the beds were laid down. 
Into these sedimentary rocks were intruded, probably in Cretaceous time, rocks of igneous 
origin. These are of two distinct types—one a diorite-porphyry and the other a quartz-porphyry. 
Both occur as sheets nearly parallel to the bedding of the sedimentaries, and as occasional cross- 
cutting dikes. The diorite-porphyry occurs chiefly as a single sheet, which widens toward the 
south and ultimately runs into the main diorite mass of the Elk Mountains. The quartz-porphyry, 
on the other hand, has probably ascended along narrow channels in the immediate vicinity of 
Aspen, and the structure of this rock shows it to belong to a type which characterizes the 
Mosquito Range, on the east side of the Sawatch, rather than the Elk Mountain district. 
Subsequent to the deposition of the Laramie and the intrusion of these eruptive rocks, 
physical disturbance began. Among the first changes was the elevation of the Sawatch Range, 
so that the beds which lay round its flanks assumed a general dip away from the main uplift. At 
about the same time occurred some minor folding, which was apparently due to a lateral thrust 
exerted from the westward, and in the Aspen district was most pronounced in a narrow zone. 
Here an overthrown anticline was formed, which culminated in a great break, called the Castle 
MON XXxXI——YIII Xxxlii 
