32 ! SCIENCE. 
the hornblende or irons in the rock, decomposes these 
minerals. Capillary attraction brings the solution to 
the surface, where the mineral matter is deposited in 
the forms mentioned. 
Careful examination, wherever exposures showed 
contacts with other rocks, failed to show any evidence 
of heat or local metamorphism of anykind. Transition 
rocks found in the immediate vicinity, as well as in 
other parts of the district, seem to furnish most of the 
links between these eruptive appearing rocks and well 
recognized metamorphic rocks. The absence of visible 
glassy enclosures, the mode of occurrence, and the fact 
that intermediate varieties occur, afford evidence that 
these rocks are simply alteration products of the gran- 
ites. They might be termed ‘‘hornblendic felstones.” 
Types of these rocks have been noticed as patches or 
masses included inthe bedding of alteration granites, 
where they have the appearance of being due to some 
econcretionary process, whereby they have become seg- 
regated into patches more basic than the balance of the 
formation. Belts of conglomerates and breccias occur 
in the district, and many good mines are found in such 
formations. The above mentioned altered granitic 
rocks comprise the most marked types, but the various 
varieties graduate one into another by innumerable 
transitions. 
Evidence of bedding is often plainly discernible in 
these rocks, but stratigraphical study is very difficult 
owing to the ‘‘wash” or ‘‘slide’”’ that covers the rocks, 
sometimes to a depth of sixty or eighty feet. Expos- 
ures are rarely to be seen except in the excavations 
made in prospecting and mining. Some of the belts of 
altered rock appear to be the upturned edges of regular 
bedded deposits, but in most cases they seem to be 
zones of alteration, and usually have a more or less north- 
erly and southerly trend. Many of these belts are 
impregnated with iron pyrites (always to some degree 
auriferous) and constitute impregnated zones similar to 
what are termed ‘‘fahlbands.” The gray pyritous fel- 
stones occur in this manner. As before stated, these 
rocks, especially where they have been leached of their 
auriferous pyrites and colored brown or yellow by the 
iron oxides resulting from the decomposition of the 
pyrites, are the rocks always spoken of in the camp as 
“porphyry.” It is not surprising that the veins occur- 
ring in these rocks are so rich in gold, but it must not 
be understood that it is the only favorable country rock 
of the district. A number of excellent mines have solid 
granite walls, and others equally good occur in the vari- 
ous formations heretofore mentioned. The disintegra- 
tion of the granites has resulted in giving the hills gen- 
tle rounded slopes, instead of the cliffs and bold escarp- 
ments such as make up the grand, precipitous scenery 
of Cheyenne Cafion and the other beautiful cafions 
around Pike’s Peak. This disintegration is going on at 
the present time in the rocks at a little depth from the 
surface. In one tunnel on the northwest slope of Bull 
Mountain, at a depth of perhaps 200 feet, and 400 or 
500 feet from the mouth, the micaless granite is found 
saturated with water. The formation is broken up 
similarly to what is termed ‘‘loose formation,’’ met 
with near the surface in most mines. The rock is, 
some of it, sorotten as to fall to pieces in handling, 
though at rest it looks to be firm, with the feldspar por- 
phyritically developed. Examination shows the feld- 
spar to be kaolinized and soft and the rock very friable. 
Two or three veins have been cut by this tunnel, and in 
these veins kaolin seams and earthy oxide of manganese 
are forming, so that one can see mineral veins in process 
of formation. Gold is found im these seemingly un- 
finished veins, but where cut by the tunnel, they are 
[Vol. XXIII. No. 572 
understood to be ‘“‘low grade.” It may be well to say 
here that in this district, as in fact in most mining 
regions, the richest ore occurs in ‘‘shoots” or ‘‘courses, ” 
and a vein may be opened by shaft, or cut by a tunnel, 
in a comparatively barren part. Drifting on such a 
vein will often develop richer ore without, necessarily, ~ 
any particular change in the outward appearance of the 
ore. In this event an ‘‘ore shoot” has been cut, which 
may extend longitudinally no great distance, but ver- 
tically, or downwards at various inclinations, it may ex- 
tend to great depths. This characteristic of mineral 
veins has been the cause of many good mines being 
abandoned, on account of cutting through an ‘‘ore 
shoot” and ‘‘losing the ore.”’ 
The broken up condition of the rock in the tunnel 
above alluded to is probably a result of the disintegra- 
tion going on, portions of the rock, for some reason, re- 
sisting the action of water, etc., better than the other 
parts, thus leaving angular or rounded fragments un- 
affected. These fragments become cemented together 
afterwards by mineral matter from solutions, and thus 
form conglomerates and breccias, such as are found in 
the belts heretofore alluded to, as well asin the mineral 
veins. 
The veins of this district are, in most cases, unques- 
tionably, true fissures, and ‘‘sheeting’”’ of the country 
rock parallel to the main fissures is a characteristic of 
many of them. The fissures seem to diverge from 
several centres in directions between 45° each way 
from north and south, and but few veins occur nearer 
east and west than 45°. Where bedding structure can 
be discerned, the veins are found to cut the bedding 
either transversely, or at greater angles from the hori- 
zontal than the dip of the beds. The fissures evidently 
did not remain open to any great width, but sufficient 
cracks remained to permit the circulation of water 
through them. Mineral solutions have eaten out the 
faces of these cracks, and replaced the eroded material 
with silica and gold in the form of vesicular or crystal- 
line auriferous quartz, together with iron and other vein 
minerals. Professor Emmons in a valuable paper 
(‘‘Structural Relations of Ore Deposits,” Trans. Am. 
Inst. of Min. Eng. Vol. XVI.) describes the parallel 
cracks which often accompany fissure veins as ‘‘minor 
fractures which form the adjacent country rock into 
parallel plates or sheets.” There are many excellent 
examples of such sheeted fissure veins in the district, 
and it islargely these quartz-filled parallel cracks that 
yield the richest ore. In many mines the highest grade 
ore is the finely divided ‘‘screenings” resulting from 
the breaking up of their quartz seams by the blasts. 
The ‘‘clay partings” are also often rich in gold. Many 
veins have only one well defined wall—the other being 
so eaten into by mineral solutions as to alter it into ore. 
Sometimes both walls are thus altered. A kaolin sel- 
vage is often found on the defined wall, and in one 
mine in particular a ‘‘slickenside,” composed of limo- 
nite, was seen taking the place of the clay selvage on a 
portion of the nearly vertical foot-wall. This ‘‘slicken- 
side” was highly polished and striated, the groovings 
dipping at an angle of about 30? from the horizontal. 
Many walls are found hardened and grooved, where a 
regular ‘‘slickenside” was impossible, on account of the 
absence of the right sort of material necessary for its 
formation. These phenomena afford evidence of a 
movement of the walls and grinding action only met 
with in true fissures. 
The veins are often mineralized to a considerable 
distance into the country rock, making it impossible to 
measure the width of the vein. In one mine belonging 
to the ‘“‘Anaconda Group” the rock was quarried out to 
