222 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
actually replaces the original minerals. Where, as in sandy dolomite, the 
original rock consisted of dolomite and quartz grains, the iron replaces the 
dolomite first, and thus cross sections of the nodules show quartz grains 
embedded in iron oxide. hese grains, however, have corroded outlines, 
and bit by bit they entirely disappear, showing that the iron also replaces 
them, although more slowly. The replacement of quartz by iron oxide in 
the process of concentration is best shown in the Cambrian quartzite lying 
close to the Castle Creek fault in Queens Gulch. Here is formed what 
appears to the eye to be a nearly solid iron ore, inclosing occasional small, 
irregular, residual portions of fine-grained, gray quartzite, which become 
stained yellow and brown and so grade off into iron. The structure of the 
rock is porous, with many small irregular cavities, the walls of which are 
lined with botryoidal or stalactitic brown limonite. Under the microscope 
the section is about nime-tenths pure iron oxide, mainly opaque, noncrystal- 
line, specular iron, having a nearly black color, with a tinge of red, and a 
metallic luster. This oxide incloses many small irregular areas of crystal- 
line quartz. Sometimes such an area is a small fragment of the quartzite, 
consisting of a number of grains, and upon the borders of this fragment the 
hematite is encroaching. The iron replaces first the secondary quartz 
which cements the grains, and so tends to surround and isolate them; but 
the irregular outlines of these separated grains show that the iron has 
affected them also, and it is evident that in course of time they disappear 
completely. This is a very satisfactory case of actual replacement. On 
the other hand, there are certain areas which are made up entirely of iron 
oxide, with no residual quartz. Instead of hematite only, there are here 
both specular iron and crystalline limonite. The limonite is translucent, 
with arich golden-brown color, and is made up of many small crystalline 
fibers or elongated plates. The specular iron and the limonite are arranged 
in beautiful concentric rings, like those of agate or Mexican onyx. These 
concentric deposits often leave an irregular cavity in the center, the out- 
lines of whose walls correspond to the shape of the concentric zones. The 
covering next these walls is generally a comparatively thick one of limonite, 
which is botryoidal in the hand specimen, and under the microscope is 
seen to be made up of long, slender, radiating crystals with spherulitic 
arrangement. This is a clear case of a filling of preexisting cavities. 
