578 DESCRIPTIVE GEOLOGY. 



tops to the north and south, forming- a rude parallelograrn, whose longer 

 axis is with the range, about 8 miles in length, and whose shorter one 

 is about 4 miles across the range, is a prominent occurrence of quartz-pro- 

 pylite. The prevailing type of rock is a fine, gray base, in which large, 

 green hornblendes and brownish-green triclinic feldspars are imbedded. It 

 shades into salmon color and green, also into drab-gray and brownish-gray 

 tones, and varies from a coarsely crystalline texture to a fine, almost felsitic 

 groundmass. The feldspars are very dull, and the hornblende more or 

 less decomposed. The groundmass presents a crystalline aggregation of 

 opaque feldspars, resembling those of granite, half-altered hornblende par- 

 ticles, and quartz which cannot be recognized by the naked eye. Under 

 the microscope, the frequent presence of these fine particles of quartz gives 

 a peculiar character to the section. The quartz contains numerous fluid- 

 inclusions, among which some salt cubes were detected. The following 

 note ^ by Professor Zirkel is of interest: 



" The large feldspar crystals are dull, but they still show that they have 

 once been triclinic, the dimming lines, densely crowded together, crossing 

 each other like lattice-work, or the bars of a grate, and leaving small, some- 

 what clearer, fields between them. The best-preserved porphyritical horn- 

 blendes are splendidly built up of long prismatic stafi"s, therein repeating 

 the peculiarity which is shown as well in the quartzless propylites as in the 

 older diorites. No dacitic hornblende has ever grown in such a manner. 

 There is apatite and also some characteristic titanite," 



An incomplete, analysis of this rock, made in Leipsic, gave : 



Silica 67.79 



Alumina 16.13 



Ferrous oxide 3.64 



Lime 2.30 



Magnesia 0.53 



Ignition 1.70 



The same great sheets of basalt which wrap around the east base of 

 the quartzite series continue to occupy the east slope of the range, and 



'Microscopical Petrography, vol. vi, 119. 



