124 DESCEIPTiVE GEOLOGY. 



associated with a decomposed mineral, which, under the microscope, looks 

 like epidote. 



Professor Zirkel has given, with some detail, in his report, the reasons 

 for regarding- these rocks as granite-porphyries. Perhaps the most import- 

 ant one, next to those already stated, the forms of the monoclinal feldspars, 

 and the occurrence of pjn-ites, may be mentioned the absence of glass and 

 the presence of liquid-inclusions in quartz-grains. This distinction, no 

 doubt, forms a sharp line, separating the two classes of rocks. Another 

 point is, tbe presence of a considerable quantity of titanite, a rare ingredient 

 in Tertiary trachytes, but nevertheless one of the component parts, of 

 trachytes from the Whitehead Peak region. 



In this connection, it is not without interest to state that, upon the 

 slopes of Whitehead Peak, is found a local occurrence of granite-porphyry 

 too limited to be indicated on the map, which is entirely surrounded by 

 trachytes. If further observations should lead to placing these rocks from the 

 North Park region among granite-porphyries, it would be necessary to 

 admit their outbreak as late as Cretaceous times, which is more recent than 

 has generally been supposed. 



The connection between the rocks of Parkview Peak region and the 

 plateau-like mass of trachytes lying to the westward was not visited by 

 our parties, the upper portion of the divide having been reached from Ada 

 Springs. The contrast in topographical outlines between the two masses 

 is very marked ; the former occurring in cones and sharp peaks, as well as 

 in long irregular dikes, while the latter is characterized by broad masses, 

 with steep acclivities, and deep mural faces. It is without doubt a trachyte, 

 although it offers some mai'ked peculiarities, which distinguish it from other 

 regions. It is a fine-grained, dark gray rock, with few crystals observable 

 to the unaided eye, except a white, glassy sanidin and minute flakes of dark 

 biotite. The mineral ingredients of the groundmass consist of sanidin, 

 augite, hornblende, biotite, and apatite. Under the microscope, the augite 

 is seen greatly to predominate over the hornblende in a sufficient degree to 

 class it as an augite-trachyte, which is a somewhat rare occurrence within 

 the belt of the Fortieth Parallel Survey. 



