SILICIFICATION. 217 
friction. The appearance, moreover, is not like that of detrital quartz, the 
grains being perfectly clear and free from any break or crack. These 
grains seem to have formed at the same time as the dolomite in which they 
are embedded, for one is often entirely inclosed in a single dolomite crystal. 
The intimate and invariable association of quartz and dolomite indicates 
that the two had a common and contemporaneous origin, and that the 
quartz grains resulted, like the dolomite which incloses them, from the 
replacement of an original calcareous sediment by silica, this silica being 
introduced into the calcareous beds by the same solution which brought 
the magnesia and produced dolomization. The theory which has been 
advanced in the case of the dolomization is that such solutions were the 
waters of a great shallow evaporating sea. 
The third process of silicification has had by far the most widespread 
effects. This silicification has come about subsequent to the formation of 
faults and fracture zones in the rocks, and is localized alone these and 
other watercourses. It is therefore distinctly later than the process which 
has formed the quartz grains in the extensive dolomite beds, by the same 
proof which shows two distinct periods of dolomization, namely, that the 
siliceous dolomite of the first period has been traversed and faulted by 
the fractures along which the dolomite and silica have subsequently been 
deposited, replacing the limestone which lies above the Leadville dolomite. 
In the earlier-formed Silurian and Carboniferous dolomites the effects of 
this later silicification are seen, although not nearly so prominent as in the 
Leadville limestone. In these dolomites there have formed, along fractures 
developed at the main period of disturbance, bands and nodules of chert. 
Such bands and nodules often follow the stratification, having been formed 
along channels parallel to the bedding. In some places there are two sets 
of chert bands developed, one following the stratification and another 
cutting across it. Such is the case in certain parts of West Aspen Moun- 
tain. Pl. XXXI is a view of a cliff of dolomite near the point of the 
mountain. The stratification planes of the dolomite dip steeply down 
toward the right-hand corner of the picture, while there is a strong vertical 
jointing running nearly at right angles to the bedding. In places this joint- 
ing becomes so prominent as to obscure the stratification. In this rock 
the chert nodules sometimes follow by preference the bedding planes and 
sometimes the joint planes. Where the jointing obscures the bedding 
