12 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
deposition and the jointing. The same phenomenon is observed in the 
Carboniferous blue limestone at Aspen, where the limestone is altered along 
joints and fault-planes to brown crumbling dolomite, locally known, from 
its finely jointed structure, as “short lime.” ‘The microscopic structure of 
the dolomite, which evidently arises from the alteration of limestone, is 
essentially like that of the dolomite of the more massive and extensive beds, 
as Sorby | has noted in the dolomites of Europe, and as the writer has found 
by a thorough study of the Aspen rocks. The structure of the Silurian 
dolomite, therefore, is one which may arise from the alteration of a calcareous 
sediment. The persistence and uniformity of this dolomite, which retains 
practically the same composition for hundreds of miles, show, however, that 
the cause of its alteration could not have been local, as in the case of the 
narrow zones which follow joints and dikes, and which were the result of 
transient circulating waters. The alteration of the Silurian dolomite from 
an assumed original caleareous sediment demands the action of waters 
throughout its whole extent for a considerable period of time. Such a con- 
dition would be afforded by the presence of a great lake or inland sea, in 
which thé usual amount of magnesia in sea water would be concentrated 
by the evaporation of the water. This is actually the condition which is 
considered by most geologists to have brought about the formation of 
nearly all, if not all, of the persistent and widespread dolomites. Dana’ 
mentions an interesting case of actually observed dolomization at the coral 
island of Metia, north of Tahiti. The rock of this island is a compact 
white coral limestone, which has nearly the composition of true dolomite, 
showing on analysis 61.39 per cent of calcium carbonate and 38.07 of 
magnesium carbonate. The general character of this rock leads to the 
inference that it was deposited from the waters of the shallow lagoon of 
the coral island in the form of fine coral mud, that the waters of the 
lagoon became concentrated by evaporation so as to contain a much 
greater proportional amount of the magnesium salts than is normal in sea- 
water, and that these solutions brought about the dolomization of the coral 
mud. The magnesium salts of the ocean consist chiefly of chloride, and 
this is quite capable of accomplishing the change in question. 
It is probable, therefore, that the Silurian dolomite of the Aspen 
district was originally deposited slowly in quiet seas, and was built up 
1 British Assoc. Report, 1856, p. 77. 2Corals and Coral Islands, p. 393. 
