DOLOMIZATION. 209 
neighborhood of the Contact fault as it outcrops on Aspen Mountain, where, 
in the blue limestone cliffs above the fault, dolomite has formed in zones 
parallel to the bedding along slips of the same nature as the Contact fault 
itself. 
The origin of the dolomite belonging to the earlier period has been 
discussed in the first chapter. It is there shown that both the Silurian dolo- 
mite and that constituting the base of the Leadville formation are contin- 
uous over wide areas, with an almost uniform chemical composition. The 
structure of the rock, as observed, is indicative of the fact that the dolomi- 
zation came about through the action of solutions, but the great extent of 
these beds shows that these solutions could not have been local or depend- 
ent upon any local cause. The agents which produced the alteration of 
the original calcareous sediments to dolomite must have operated equally 
over the whole area in which we now find these beds dolomized. Such 
widespread solutions would be afforded by the presence of a great lake or 
inland sea in which the usual amount of magnesia in sea water was concen- 
trated by evaporation. 
The origin of the dolomite belonging to the later period, however, 
was evidently distinct from that which has been suggested for the extensive 
beds. As before, the microscopic structure shows that the rock was 
produced by the action of solutions which exchanged carbonate of mag- 
nesia for part of the original carbonate of lime, but in this case the 
solutions have been comparatively scanty and short-lived, for they have 
not affected the whole rock, but only zones along watercourses. In the 
extensive beds of dolomite belonging to the earlier period the change is 
practically uniform and complete. This has been proved in the case of the 
Leadville dolomite by a number of analyses, such as are given in Chapter 
I, p. 25. These analyses show a comparative uniformity throughout the 
rock, and show that the whole formation is nearly a true dolomite, having 
a slight excess of lime. This dolomite, as also that which originated later, 
always contains some iron and silica, which were introduced probably at 
the same time as the magnesia. On the other hand, analyses of the locally 
dolomized zones which traverse the blue limestone show all transitions 
from a pure dolomite into a pure limestone, the dolomization growing less 
as the distance from the fracture along which the dolomizing solutions 
apparently flowed increases. To test this fact, analyses were made of rock 
MON XXXI——14 
