FERRATION. 221 
comparative analyses which were cited to prove this phenomenon at Glen- 
wood Springs also show an accompanying silicification, which is actually 
being brought about by the ascending hot waters. 
The effect of silicification along fault planes is seen not only in dolo- 
mite and limestone, but to less extent in other rocks. In the calcareous 
Maroon sandstone at the Yopsie tunnel, in Queens Gulch, the chief altera- 
tion accompanying the formation of barite and other gangue materials 
appeared, under the microscope, to be the accession of silica. This silica 
sometimes penetrates the rock along porous zones, where the original frag- 
ments are cemented by secondary silica, as in quartzite; or, in the more 
altered portion, the rock is recrystallized so as to resemble jasperoid. In 
granite lying against the Castle Creek fault in Queens Gulch the original 
quartz grains have been enlarged by secondary silica so as to present 
hexagonal idiomorphic outlines. These enlarged crystals have zonal 
structure, showing their method of growth. i 
FERRATION. 
Another widespread chemical change in the rocks is the deposition of 
iron. This is frequent in limestone, where the alteration into iron has 
taken place along water channels, thus. showing the secondary nature of 
the mineral. Almost invariably the ferration is only partial, and often 
extremely slight, accompanying dolomization and _ silicification. Along 
zones where these processes occurred the iron seems to have been origi- 
nally deposited chiefly as carbonate, which crystallized together with the 
dolomite. Often, however, the microscope shows small crystals of pyrite 
embedded in ferriferous dolomite, and their relation is such as to indicate 
that sulphide and carbonate of iron have erystallized at about the same 
time, for the sulphide is confined to those areas where the calcium car- 
bonate has been replaced by magnesium and iron carbonate. On oxidation 
the iron carbonate changes to oxide, thus giving the brown color which all 
oxidized dolomite has in this district. This oxidation brings out the iron 
in the rock more plainly under the microscope than when it is in the form 
of carbonate; and all the stages of change can be seen in partially altered 
specimens. After oxidation the iron shows a tendency to segregate, and so 
form nodules, and in the process of formation the iron of these nodules 
