216 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
cold springs which carry a large amount of sulphureted hydrogen and 
which are apparently ascending. We may conclude, then, that the later 
period of dolomization at Aspen was probably due to the effects of hot 
springs rising along faults and fractures. These altered the limestone to 
dolomite by means of the carbonate and chloride of magnesia which they 
they held in solution. 
SILICIFICATION. 
Among the chemical changes which have taken place in the Aspen 
rocks since their formation the deposition of silica has been one of the most 
noteworthy. The processes of silicification may be divided into three. 
First, the conversion of sandstones to quartzite by filling of the interstitial 
spaces; second, the deposition of silica at the same time with carbonate of 
magnesia from oceanic or lake waters; third, the deposition of silica along 
fault and fracture zones and other watercourses subsequent to the disturb- 
ance in the rocks produced by faulting. 
As an example of the process first named, the Cambrian quartzite is 
prominent. This has been formed from original sandstone by growth of 
the detrital grains of which the sandstone was formed, after the manner 
of crystals, the silica to supply this development being furnished by waters 
which penetrated the porous rock. In the Parting Quartzite series the same 
process has gone on, although not always completely. Inthe Dakota sand- 
stone there are certain interesting phenomena attending the early stages in 
the change of the sandstone to quartzite. This formation is mostly sand- 
stone, but contains thin intermittent bands of true quartzite. Often the 
quartzite is not even banded, but is in irregular nodules distributed along 
‘certain zones. The extension and consolidation of these bands and nodules 
would make the whole formation a quartzite similar to the Cambrian. 
The second process of silicification is illustrated in the dolomite of the 
Silurian and the Lower Carboniferous. These dolomites are always some- 
what siliceous, and the microscopic examination reveals the presence of 
quartz, which is generally evenly disseminated in small grains of varying 
size. On casual examination these grains appear detrital, but when care- 
fully observed, instead of being rounded or even regularly angular, they 
are seen to be irregular and sinuous, presenting reentrant angles and sudden 
bays, such as could not possibly appear in grains which had suffered any 
