100 DESCEIPTIVE GEOLOGY. 



gradually toward the higher portions of the range. On the west side, there 

 are two prominent hills, which, from their isolated position, serve as land- 

 marks in the valley. Bennett's Peak, opposite the mouth of Brush Creek, 

 rises 600 feet, and River Butte, 5 miles below, at the mouth of Grand En- 

 campment Creek, 900 feet above the river-bed. They are both made up 

 of very dark, heavily-bedded hornblende-gneisses, dipping to the westward 

 at a high angle. They are probably metamorphic diorites. 



Between the Platte Canon and the Laramie River, the country is either 

 covered with a dense growth of forest, or else with soil and detrital matter, 

 to such an extent that geological relations are difficult to make out. Along 

 the Laramie and North Park road the rocks are chiefly coarse-grained, but 

 showing more or less bedding. Hornblende appears to be a constant con- 

 stituent, and the feldspars are usually light-colored. Between French and 

 Brush Creeks, mica-bearing rocks prevail. A typical specimen, collected 

 on the flat-topped ridge 2 miles north of French Creek, is a light-colored 

 rock, crumbling easily, with a coarse, sandy texture. It is made up of 

 limpid quartz, both monoclinic and triclinic feldspars, and small, minute 

 flakes of dark mica. No hornblende was noticed, and our observations tend 

 to show that it is somewhat remarkable to find mica present with so much 

 plagioclase, and free from hornblende. The mica occurs in narrow layers, 

 giving the rock a finely-banded appearance, but with the evidences of bed- 

 ding much less marked than is usually seen in this region. The strike was 

 from north 45° to 55° west. In this light mica-gneiss occurs interstratified a 

 narrow belt, only a few feet in width, of diorite, which stands up prominently 

 above the surrounding rock, and has much of the habit of an intrusive dike. 

 In many respects, however, it closely resembles the finer-grained varieties 

 of the metamorphic diorites ; but a careful study of a specimen indicates 

 certain mineralogical differences that have not been detected in the clearly 

 metamorphic dioritic schists of the region. To the unaided eye, the rock 

 presents an irregular admixture of a mass of hornblende and feldspar, of a 

 dark gray color, with only a few long, acicular crystals of striated feldspar. 

 Under the microscope, a thin section shows only hornblende, plagioclase, 

 and a few orthoclase crystals, but no mica, quartz, or apatite. Zirkel calls 

 attention to the singular arrangement of the groundmass. 



