434 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
of this report. It is normally rather coarse grained, with conspicuous porphyritic 
crystals of pink microcline. A tendency toward gneissic structure is frequently 
noticeable. 
The common breccia of the Portland mine is a hard, rather even-textured 
gray rock in which the breccia structure is seldom very conspicuous. Recognizable 
fragments of phonolite or andesite over an inch in diameter are rare, and the breccia 
in which occur the ore bodies of the Captain and Hidden Treasure veins, near the 
No. 2 shaft, is so fine grained and homogeneous that its true character might easily 
escape detection by an eye untrained to its peculiarities or familiar onl} 7 with the 
more obvious nature of the breccia prevailing in the Raven Hill mines. Locally, 
however, blocks of granite, some of them as much as a foot or even 6 feet in diameter, 
occur within the breccia, particularly in the vicinity of the contact with the granite 
wall which limits the breccia on the south. This granitic breccia is very well shown 
on the 500-foot level, in the west crosscut to the Bobtail vein, northwest of the 
Burns shaft. Although such large fragments of granite are not common in the 
breccia of the northern part of the mine, yet close inspection of the finer grained 
phases, such as constitutes the country rock of the Captain veins, usually reveals 
little bits of pink microcline derived from the comminution of the granite. The 
microscope shows that together with particles of microcline and quartz of granitic 
derivation occur splinters of plagioclase and small fragments of the feldspathic ground- 
mass of andesitic and phonolitic rocks. No ferromagnesian minerals have been 
seen in any of the volcanic breccia of the Portland mine. Secondary pyrite is 
almost everywhere present, disseminated in varying abundance through the rock 
and usually accompanied by more or less calcite. 
In certain parts of the mine the breccia exhibits banding, which in some cases 
closely resembles ordinary bedding. Such a structure was noted on the 600-foot level 
east of the No. 2 vein, in a tine-grained phase of the breccia resembling a tuffaceous 
sandstone. The bands here are rather indistinct and are due to not very sharply 
differentiated alteration of slightly coarser layers with the prevailing fine grained 
material. Similar banding occurs in the northern part of the mine on the 500-foot 
level in the country rock of some of the Captain veins. The bands here are gener¬ 
ally nearly horizontal. They are not persistent, but fade out gradually into the 
fine-grained homogeneous breccia. The structure is best exhibited, however, south 
of the Burns shaft on the 220-foot level. Here also the banding occurs in an unus¬ 
ually fine-grained tuffaceous phase of the breccia which grades both horizontally 
and vertically into coarser material showing no banding. The bands are a foot or 
two thick and are nearly horizontal. The rock cleaves parallel to the bands and 
thus heightens the effect of regular stratification. The banded structure, however, 
is very local and passes by insensible gradations into coarser breccia, in which no 
bonding is perceptible. 
The origin of this handing in the tuff breccia is not clear. There is no evidence 
of the former existence of any considerable water body in which stratification might 
have taken place, nor do the constituent particles of the breccia appear waterworn 
when examined in thin section. It is probable that the structure is a very local 
phenomenon, incidental to the accumulation of the volcanic ejectamenta, and that 
it is to be ascribed to the winnowing action of air currents on fragments thrown up 
