584 DESCRIPTIVE GEOLOGY. 



Lower Quaternary, as laid down by us, is not necessarily the upper part of 

 the Pliocene. 



Region of the Clueo Hills. — On the west of the northern portion of 

 the range, a small, detached, outlying group of hills, called the Cluro Hills, 

 rises out of the plain, edged by Quaternary on the east and by a series of 

 flanking deposits of Pliocene on the west. The hills themselves are com- 

 posed of syenite, granite, and a quartzite body which has been, for conven- 

 ience, referred to the Weber group. These hills are of interest as showing 

 the only true old syenites of the collection. This rock consists of a flesh- 

 colored monoclinic feldspar and greenish hornblende. Under the microscope, 

 the feldspars resemble the orthoclase of granite. The hornblende proves 

 not to be made up of homogeneous individuals, but every one of an associa- 

 tion of light-green rays or prisms. This polysynthetic arrangement sug- 

 gests the hornblendes of propylites. The rock also contains microscopical 

 quartz, and a little plagioclase feldspar, whose striation is, however, very 

 indistinct. Both these minerals have been generally considered not to occur 

 in syenites, and their presence in this rock could not be detected but by 

 the aid of the microscope. The granite of the Cluro Hills is similar to that 

 already described in the Cortez Range. 



To the north of Wagon Canon, dacite forms the main crest of the 

 Cortez Range, to the west of which is a body of hornblende-andesite, whose 

 outcrops are somewhat obscure, and only exposed by erosion. This dacite 

 diifers somewhat from that to the south of Wagon Canon in being less of a 

 brecciated mass, though it still includes some breccia fragments. It is of a 

 purplish-green color, and contains large, opaque, tric'linic feldspars, with a 

 good deal of fresh, brownish hornblende, and some mica, in a micro-felsitic 

 groundmass. The quartz crystals are not apparent to the naked eye, but 

 imder the microscope can be easily distinguished. The feldspars contain 

 inclusions of a yellowish-gray glass, which sometimes almost replace the 

 entire felspathic substance. The hornblendes are frequently decomposed, 

 being altered into a pale-green substance, which sometimes has a mixture of 

 calcite, but the biotites are always fresh. The andesite which appears on 

 the slopes of the range toward the Cluro Hills is a dark, even-grained 



