CARBONIFEROUS FORMATIONS. 25 
that they are inclosed in crystals of dolomite, that they have formed subse- 
quent to the deposition of the rock. These grains are often clustered in 
certain areas, displacing the dolomite and forming chert nodules and bands. 
chert—The chert is generally dark gray in color, sometimes light gray. 
It is in seams, nodules, or bands, which usually follow the bedding, but 
occasionally, as on West Aspen Mountain, the seams follow a set of 
fractures which cut across the bedding, and so give a deceptive appearance 
of stratification. 
Composition of Leadville dolomite —Many analyses of the dolomite have been 
made by the mine managers of Aspen, who consider the determination of this 
rock an important aid in prospecting for ore; and these analyses have been 
supplied to the Survey by the courtesy of these gentlemen. The following 
eighteen show the respective proportion of oxide of magnesium and oxide 
of calcium in as many different samples. These were selected at random 
from the list, but care was taken to include only those which were from 
the dolomite stratigraphically below the blue limestone, and to exclude 
dolomite which is a local alteration of the limestone. This latter dolomite 
shows transitional stages from the limestone which are indicative of its 
origin, while the composition of the dolomite which everywhere underlies 
the blue limestone is nearly uniform. J ollowing is the table: 
Oxides of calevwm and magnesiwm in Leadville dolomite. 
CaO MgO 
33.4 23.2 
30. 46 20.9 
Bieey 16.57 
32.5 17.72 
34.9 15. 62 
31.7 15.83 | 
31.6 17.80 
31.3 19. 20 
7 13.29 
34.2 12.97 
30.2 20.80 | 
31 20.60 | 
30.4 19.46 | 
32.2 17.90 
30.9 20.40 | 
31.3 17. 83 
35.2 16.21 
30.9 20. 4 
Average_32.3 18.15 | 
