XXXIV OUTLINE OF THIS MONOGRAPH. 
Creek fault. Probably beginning at the same period, but continuing afterwards, was the develop- 
ment of a domelike uplift, which affected both granite and sedimentary rocks in a restricted 
region east of the Castle Creek fault, now occupied by Aspen Mountain and Tourtelotte Park. 
This uplift was marked on the north side by asharp bending-up of the strata, while on the west 
side the movement took place along the previously formed Castle Creek fault; on the other side 
the extent of the uplift can not be accurately judged, on accouns of its running into the granite. 
The bending-up of the beds was accompanied by faulting, which has gone on continuously from 
that time to the present day. At the beginning few faults were developed, but these appear to 
have had an important throw. As erosion progressively removed the overlying load of strata, 
the faults became more numerous and complicated, but the amount of throw in each case grew 
less, A number of distinct fault systems have been identified, differing chiefly in point of age. 
This difference in age is shown by the faulting of one system by a later system, and also by the 
fact that certain faults have developed before and certain others after the ore deposition. It is 
also shown that some faults have developed almost entirely in post-Glacial time, and that in 
many cases the fault movement is going on at the present day. 
Along the channels afforded by faults hot-spring waters rose and brought about certain 
chemical changes. One of the most interesting of these is dolomization, and the combined 
evidence at Aspen and at Glenwood Springs, where the change is now being brought about by 
hot ascending waters, shows that the process is essentially a chemical interchange effected 
between the calcium carbonate in the limestone and carbonate of magnesia brought in by these 
circulating waters. Thus zones in the limestone following watercourses which are parallel to 
the bedding or which cut across it are locally altered to dolomite. There is, however, evidence 
of an earlier period of dolomization, which preceded the faulting and probably came about very 
early in the history of the rocks. Thus the Silurian sediments and those of the lower part of 
the Leadville formation were early converted into dolomite, probably by the action of magnesium 
salts contained in the waters of a shallow and evaporating sea. 
Associated with the formation of the dolomite along fault fractures and watercourses is 
the deposition of silica and of iron, and both these processes must be referred to the same 
cause as the dolomization. 
The ores of the district consist chiefly of lead and zinc sulphides, carrying silver, with a 
gangue of barite, quartz, and dolomite. On oxidation the sulphides change to sulphates, carbon- 
ates, and oxides. The deposition of the metallic minerals has taken place almost exclusively along 
the faults, but it is only in certain places that the fault zones become sufficiently mineralized to 
form valuable ore, for it is chiefly at the intersection of two or more faults that rich shoots are 
formed. The intimate association of the metallic sulphides with dolomite, quartz, and other 
gangue materials suggests a common origin for all—that they were deposited by ascending hot 
waters. Since the ore has been found chiefly at the intersection of faults, the theory is advanced 
that solutions ascending along one of these channels were precipitated by solutions which 
circulated along the other. 
Among the more recent chemical changes in the rocks, mainly subsequent to the ore depo- 
sition and its attendant phenomena, is the formation of sulphates. Thus a considerable quantity 
of gypsum has been locally precipitated, and soluble sulphates occur as incrustation on rocks 
which have been exposed to oxidizing influences. By a process of reduction there has also been 
locally formed a large amount of native silver. 
