94° +GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
blue limestone, through the action of magnesian salts, which were held in 
solution in the waters of a probably evaporating and shallowing sea that 
covered the whole district. 
Description. —W here the Carboniferous dolomite has not suffered oxidation 
it is hard and gray blue in color, compact, and with a rough, conchoidal 
fracture. Such fresh rock, however, is found in large quantities only m 
the lower levels of some of the mines, such as the Free Silver and the 
Smuggler, in which, 700 or 800 feet below the surface, a depth is reached 
where the effects of oxidation have been little felt. In these mines the 
changes which one observes in the dolomite, going upward toward the sur- 
face, explain the whole process of alteration. The‘rock becomes yellowed 
by the formation of a small amount of iron oxide, which microscopic study 
shows to be probably derived from the oxidation of iron carbonate that 
is crystallized with the dolomite. The alteration of the carbonate to the 
oxide is accompanied by a decrease of bulk, and some dolomite is also 
probably carried away in solution by percolating carbonated waters; these 
withdrawals bring about concentration, which results in the formation of 
numerous joints. These joints become so close that when the rock is 
struck forcibly it often crumbles into many small, angular fragments. 
From its color the oxidized dolomite is called by the miners “ brown 
lime,” ' while from its close jointing it is called “short lime.” ‘These pecul- 
iarities are characteristic of the rock to a greater or less extent all over the 
surface and within the zone of active surface alteration. 
Microscopic structure—The structure of the Carboniferous dolomite is iden- 
tical with that of the Silurian The rock is made up of small, gray 
erystals of dolomite, interlocking, and with a constant tendency to rhom- 
bohedral form. These erystals are usually uniform im size, but sometimes 
they vary slightly in different areas, which change gradually one into the 
other. On oxidation they develop iron oxide along their edges and in 
cleavage cracks, showing that they contain a small percentage of iron 
carbonate. Quartz is always present in the same peculiar grains which 
have been noted in the Silurian dolomite, and which are easily taken for 
detrital grains, but which on close examination show by their fresh, 
unbroken structure and irregular outlines, as well as by the circumstance 
limestone or to the blue lime, which has been greatly altered to a brown lime that is not dolomitic, 
but is a porous lime carbonate containing much iron. In this paper ‘‘ brown lime, ” «‘ short lime,” 
and ‘‘ dolomite” are synonymous. 
