98 DESCRIPTIVE GEOLOGY. 



abundant in the Colorado Range, does not appear to be present. In this 

 rock, the quartz and feldspars are intimately associated together, with some 

 thin flakes of mica adhering to the feldspar crystals, in bands of greater 

 or less fineness. The mica, which is quite prevalent, occurs in well-defined 

 layers. 



Clark's Peak is a mass of true granite. It differs from nearly all the 

 rocks of the range in showing but little, if any, tendency to bedded 

 structure, and no parallelism in the arrangement of the constituent minerals. 

 The rock in many respects resembles an intrusive body, and such, on further 

 observation, it may prove to be. It is compact, moderately fine-grained, 

 with no large crystals porphyritically enclosed, and resists atmospheric 

 agencies remarkably well. In color, it is pearl-gray. Like the other 

 rocks of the region, it is rich in triclinic feldspars. The quartz occurs in 

 limpid white grains. Both orthoclase and plagioclase are present; the latter 

 have a brilliant lustre and present the characteristic striae well marked. 

 The mica, probably a biotite, is finely disseminated through the mass with- 

 out any apparent order. Hornblende, which is present in so many of the 

 rocks of this range, is not observed in the specimens obtained on Clark's 

 Peak. 



Under the microscope, apatite may be seen, but no titanite; and the 

 quartz carries many liquid-inclusions. 



This rock is one of the finest granites in our collection, and would 

 make an admirable building -stone if it were only accessible. In the imme- 

 diate region of Clark's Peak there are some similar-appearing granites, 

 which probably belong to the same formation, and it might be possible to 

 trace their connection. In one, 3 or 4 miles southwest from the peak, near 

 the base of the range, Zirkel detected the presence of zircon in red grains, 

 closely resembling the minute zircons found, in the syenites of Norway. 

 The granite is lighter-colored than the Clark's Peak variety, and carries 

 vitreous oligoclase crystals, an half-inch in length. 



Due west from Clark's Peak, and separated by a low divide and two 

 glacially eroded canons, stands a prominent peak of gneissic rocks. Near 

 the base occurs a typical variety of the region, — a fine-grained compact 

 gneiss, composed of mica and orthoclase with much hornblende and plagio- 



