320 DESCRIPTIVE GEOLOGY. 



altered hornblende, and less frequent crystals of plagioclase, in a compact, 

 reddish, semi-vitreous groundmass. A still more beautiful rock in the same 

 locality is of a bright grass-green color, containing the same predominant 

 mass of sanidin crystals, with fresh unaltered hornblendes, the crystals of 

 Avhich, under the microscope, are seen to be ruptured in a most remarkable 

 manner. On the walls of the pores or cavities of the rock, the microscope 

 discloses a white siliceous deposit in stalactitic forms, which also penetrates 

 the groundmass, as well as the feldspars and hornblendes, in small veins. 

 Zirkel suggests that, as no tridymite is found in these rocks, this siliceous 

 deposit may be considered to take its place. The groundmass is seen to 

 contain some micro- felsitic base. The former of these trachytes is remark- 

 able from the fact that the plagioclase crystals are seen to contain a moving 

 bubble within the glass-inclusions, like the leucite lavas from Capo di Bove 

 and Solfatara in Italy. This bubble was not absorbed when heated to 120°. 

 An analysis of the green trachyte, made by Mr. R. W. Woodward, 

 gave the following result: 



SiHca : 64.82 64.93 



Alumina '. 15.37 15.38 



Ferric and ferrous oxides 5.11 5.10 



Manganous oxide trace trace 



Lime ■ 4.90 4.90 



Magnesia 2.22 2.24 



Soda 2.65 2.55 



Potassa 3.03 3.05 



Litliia trace trace 



Water , 2.18 2.14 



100.28 100.29 



The trachyte from Silver Creek Canon is a gray, rather finer-grained 

 rock, rich in crystals of sanidin, plagioclase, and fresh brown hornblendes; 

 the latter in needle-like prisms, often half an inch long. It also contains 

 biotite, with augite, and occasionally tridymite. Under the microscope, the 

 biotites are seen to be perforated by numerous apatites, and the feldspars 

 to be rich in glass-inclusions. 



