608 DESCRIPTIVE GEOLOGY. 



limestones, whicb are here exposed by the erosion of the ravine. The 

 diorite is a fine-grained, dark-green variety, in which but few macro- 

 scopical crystals can be distinguished. It is made up mostly of plagioclase, 

 mica, calcite, and a dark-green product of alteration, which Professor Zirkel 

 thinks should be referred rather to augite than to hornblende, which would 

 place the rock among the diabases, although, from a macroscopical examina- 

 tion, it has been colored as a diorite. The line of the upper geological 

 section at the bottom of the map cuts the range at Taylor's Creek, show- 

 ing this dike, and the fault on the western slopes. To the southwest of Tay- 

 lor's Creek, the low ridge which divides Independence Valley from Maggie 

 Creek Valley is mostly occupied by flows of rhyolite, generally of white and 

 red porphyritic varieties, which, as shown by the few outcrops and quartzite 

 ddbris found in the ravines, have covered a pre-existing ridge of Weber 

 Quartzite. 



Northern Cortez Eange. — That portion of the Cortez Range which 

 bounds Maggie Creek on the west, and is connected by this ridge with the 

 Seetoya Mountains, for the most part has not been covered by the rhyolite 

 flows which have deluged this region. These hills are, however, remark- 

 ably thickly covered by detrital material, and show very few outcrops. 

 From the not always reliable inferences to be drawn, from fragments found 

 in the ravines, the hills are mainly composed of quartzites, generally white, 

 compact, and somewhat iron-stained varieties. At the Dalton Peaks are 

 found almost the only good exposures of the sedimentary rocks, which make 

 up the range. Here the eastern peak shows quartzites and siliceous shales, 

 through which runs a black cherty vein, striking due north, and dipping 

 25° to the eastward, while on the western peak are the peculiar greenish 

 quartzitic conglomerates characteristic of the Weber Quartzite. These 

 quartzites are very heavily bedded, and stand at a very steep angle, appa- 

 rently with a slight dip to the westward. The structure here, then, as well 

 as can be arrived at, is an anticlinal, whose western member is steeper, and 

 has perhaps suffered displacement. 



In the valley of Boulder Creek, to the west of Dalton Peaks, at 

 White's Ranch, is a little isolated hill of limestone, overlaid by a greenish- 

 white quartzite, from which were obtained a collection of Lower Holder- 



