AREAL GEOLOGY OF LOWER CLEAR CREEK 275 



DiORiTE Pegmatite. Guy Gulch, by C. & S. tracks 

 Macroscopic. — The hand specimen is a rather pegmatitic -looking mass composed 

 entirely of hornblende, feldspar in most cases striated, and black mica. 



Microscopic. — Under the microscope the rock is seen to consist of a coarse-grained 

 aggregate of green hornblende, biotite, plagioclase, orthoclase, the former predominating, 

 together with a httle quartz, apatite, epidote, calcite, titanite, and magnetite, all exhibiting 

 the ordinary characteristics of these minerals. The quartz and feldspar show frequent 

 pegmatitic or granophyric intergrowths. The plagioclase appears to have a low extinc- 

 tion angle, and to be near the albite end of the series, probably oligoclase. 



Latite Porphyry 



In certain portions of the district in question, notably the areas covered by the special 

 maps, and also in the western portion of the Black Hawk Quadrangle, the country is 

 extensively traversed by porphyry dikes. While these dikes run in every direction, the 

 majority of them have a course N. E. by S. W., or the same as the prevaihng vein system. 

 It is interesting to note that, while there are no veins of importance outside of the area 

 traversed by the dikes, the area of the dikes at the same time extends beyond the area 

 traversed by the important veins. These dikes vary in size from a few inches to hundreds 

 of feet in width, and from a few feet to many miles in length. As a rule, however, they 

 die out in a few hundred feet. In the field they all have the same general characteristics, 

 and vary in color from brown and red to gray. They are reasonably straight, but dip at 

 all angles, though, like the veins, in the majority of cases the dip is rather steep. 



At various points all over the area studied, and especially near veins, the dikes have 

 been brecciated, and in some cases the fragments in every way resemble water-worn 

 pebbles. These occurrences have been explained as true conglomerates of as early as 

 Algonkian time, notwithstanding the fact that there is good reason to consider the por- 

 phyry composing the pebbles as Tertiary. The phenomena have also been explained as 

 resulting from the fall of water-worn pebbles into open crevices. The true explanation of 

 the phenomena probably is the extensive brecciation and complete shattering of the dike. 

 Subsequent kneading movement, possibly aided by solution, reduced the angularity of 

 the fragments, and in some instances gave them the appearance of water-worn pebbles. 

 The best places to study these phenomena are Banta Hill and the so-called "Queen's 

 Chair." 



The dikes, as a rule, are closely "frozen" to the inclosing walls, and, though carefully 

 sought for, in but one case was any sign of contact metamorphism observed. That there 

 has been shrinking in cooling is evident by the lines of fracture when the rock is broken. 



It is difficult to obtain fresh specimens on the surface, and when near veins they are 

 also greatly decomposed, as a rule. The best specimens are to be procured in cross-cut 

 tunnels outside of the vein area and in newly made roads. The dikes usually have a fine- 

 grained appearance when fresh, and the porphyritic appearance and the display of pheno- 

 crysts, as a rule, are the result of weathering or underground decomposition. In the hand 

 specimen the rock varies through all the stages from what might be called a felsite, which 

 is all ground-mass with no phenocrysts, to a granite porphyry which has little ground-mass, 

 and which might be said to be all phenocrysts. This latter phase is often taken for a 

 granite and not regarded as a porphyry at all. Those rocks locally known as porphyry 



