26 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
The average of all these analyses gives 32.3 per cent of calcium oxide 
and 18.15 per cent of magnesium oxide. Since they are taken from many 
parts of the district, their average may be accepted as the composition of 
the dolomite constituting the lower part of the Leadville formation. 
Normal dolomite, according to Dana, contains 45.65 per cent of 
magnesium carbonate, and 54.35 per cent of calcium carbonate. ‘This is 
equivalent to 21.74 per cent of magnesium oxide, and 30.43 per cent of 
calcium oxide. The Leadville dolomite of Aspen, therefore, contains some- 
what more lime and less magnesium than typical dolomite. There is also 
nearly always present a small amount of silica and of iron. Not so many 
determinations of these ingredients were available, but from the average of 
several analyses of each it is found that the silica constitutes probably 
Fiqa. 4.—Sandstone veins in dolomite. 
2 or 3 per cent and the iron oxide about the same. The iron oxide deter- 
mined by the analysis, it will be remembered, is really in the form of 
carbonate in the unaltered rock. There is also often present a small 
amount of alumina. 
Sandstone veins in dolomite —Mr. Tower observed on Aspen Mountain irreeu- 
lar sandstone veins of small size in the dolomite. ‘These veins were often 
conformable to the bedding, but were also found cutting vertically across 
the bedding and connecting with the horizontal veins, so that it was evident 
that they were of later origin than the inclosing rock. The finding of angu- 
lar fragments of the dolomite among the material of the vein confirmed 
this conclusion. 
The largest of the veins observed was only a few inches wide, and from 
that they grade downward in size, sometimes filling crevices which measure 
