208 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
under such circumstances. The ordinary reef-forming corals, however, 
contain. very little magnesia, and therefore the formation of this dolomite 
seems to be connected with the peculiarities of the lagoon in which it is 
deposited. Such a landlocked lagoon is a very favorable place for the 
evaporation of water and the concentration of the salts which it contains, 
and these concentrated salts of magnesia are supposed to have acted upon 
the coral so as to form dolomite. In the same way dolomite may be formed 
over wide areas where a previously deposited limestone is covered by the 
shallow waters of a landlocked and evaporating sea. 
On a more local scale the alteration of limestone to dolomite has been 
observed by various writers. Such alteration along joints in the Carbonif- 
erous and Devonian limestones near Cork, in Ireland, has been described 
by Professor Harkness.! 
The nature of the exact chemical process in this alteration has not been 
definitely agreed upon, the chloride, the carbonate, and the sulphate being 
variously considered as the possible magnesian salt which has reacted upon 
the carbonate of lime. More recent investigations, however, seem to show 
that any or all of these salts may have this effect. The general chemical 
reason for the formation of dolomite seems to be the tendency of the car- 
bonates of magnesia and lime to unite to form a double carbonate. 
ASPEN DOLOMITES. 
In Aspen there is evidence of two distinct periods at which dolomite 
was formed. The main body of dolomite, consisting of the whole of the 
Silurian beds and the lower part of the Leadville formation, existed previous 
to the deformation of the rocks by faulting and folding, since the dolomite 
beds are faulted in the same proportion as the rest of the strata. Along 
these main faults and fractures, however, the blue limestone at the top of 
the Leadville formation has been altered irregularly for varying distances 
on each side of the fractures into dolomite, microscopically identical with 
that formed at the earlier period. This later dolomite forms wherever -the 
rock has been open to circulating waters. Thus it follows faults and frac- 
tures which cut directly across stratification, and also slip planes and porous 
zones conforming with the stratification. In the latter case the dolomite 
forms apparent beds in the blue limestone, as is especially well seen in the 
1Quart. Jour. Geol. Soc. London, Vol. XV, 1859, p. 101. 
