CH AE ip Live 
CHEMICAL GEOLOGY. 
In this chapter the various chemical changes which have occurred in 
the rocks of the Aspen district since their -consolidation will be discussed. 
Nearly all of these changes have come about since the beginning of deforma- 
tion in the beds, and they have apparently an intimate connection with the 
physical changes, such as faulting and folding, which have been described 
in Chapter II. From an economic point of view the most important of the 
chemical changes was that resulting in the deposition of the precious metals, 
but certain other changes are much more widespread, and thus are more 
important geologically. These latter changes consist in the alteration of 
limestone to dolomite by a process of replacement, and, by other processes 
of replacement, in the alteration of limestone and dolomite, but chiefly the 
former, to quartz and tc iror. These processes of dolomization, silicifica- 
tion, and ferration are all intimately connected with the deposition of the 
precious metals, but are more widespread, as might be expected from the 
great amount of magnesia, silica, and iron throughout the crust of the earth 
as compared with the smaller amount of the rarer metals. All these chem- 
ical changes have occurred mainly at the same period and apparently 
through the same agents, but the process of ore deposition requires more 
peculiar and unusual conditions than do the others, and therefore probably 
acted during a shorter period than they. 
DOLOMIZATION. 
As seen under the microscope, all the dolomites of Aspen have essen- 
tially the same structure, and this structure is clearly not that of a sedimen- 
tary deposit. The dolomite crystals which make up the larger part of the 
rock are all apparently crystals of the double carbonate and occur mostly 
in nearly perfect interlocking rhombohedra, and associated with these are 
invariably small grains of quartz whose character and shape show that they 
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