684 DESCKIPTIVE GEOLOGY. 



a uniform fine-grained groundmass, but with brilliant particles of iron 

 pyrites scattered through the rock. Another dike exposed in the steep 

 walls of Dale Cafion presents a rather striking contrast with the one just 

 described, having a light-green color, owing to the feldspar greatly pre- 

 dominating over the hornblende, which occurs in altered fibrous needles. 

 There are some quartz-grains scattered through' the groundmass. Under 

 the microscope may be detected apatite in slender needles. Under the ham- 

 mer, these rocks all exhibit much the same character, breaking with difficulty 

 in a manner indicating a dense tough groundmass rarely met with in Tertiary 

 volcanic products, and quite unlike the microfelsitic rhyolites, or rocks like 

 the basalts rich in amorphous glass-base. It is impossible for one familiar 

 with the habit of rock-masses, and who has studied them in the field, not to 

 recognize marked distinctions in the behavior of rocks under the blows of a 

 hammer, distinctions which may be subtle and difficult to explain, yet sub- 

 sequent analj^tical work shows that they were based upon fundamental differ- 

 ences in the rocks, and that the field impressions were correct. 



To the Tertiary volcanic rocks of the Havallah Range, frequent allu- 

 sion has already been made. They consist mainly of rhyolites, but there 

 is one outburst of trachyte that calls for some special mention. It is situ- 

 ated just south of Gold Hun Creek, high up in the range, at the junction of 

 the older quartzites with the Star Peak beds, where it forms a north and 

 south ridge 3 or 4 miles in length, with several peaks and cones rising above 

 the general level. The inclination of the mass is to the eastward. The 

 rock is a quartz-bearing trachyte. It possesses a light-gray color, a highly 

 porous trachytic texture, and closely resembles the typical variety from 

 the famous Drachenfels on the Rhine. The sanidin crystals are finely 

 developed, frequently an inch and one-half in length, with broad tabular 

 faces, but most of them are shattered and broken. Minute flakes of dark 

 magnesian mica are everywhere scattered through the groundmass. The 

 quartz appears to be segregated in large macroscopical grains as an acces- 

 sory mineral. 



The main bodies of rhyolite are found at the extreme northern and 

 southern ends of the range, coming to the surface where the long uplifts of 

 Mesozoic strata plunge rapidly downward. Here, as if at points of greatest 



