THE NOKTH PARK. 123 



iron minerals, magnetite, and pyrites, the latter, on exposure, falling out 

 from the rock. It is an exceedingly tough, dense mass, breaking with a rough, 

 hackly fracture and angular edges. In its mineral composition, this rock, 

 both macroscopically and microscopically, is exceedingly rich in species, 

 the following being recognized by the eye : orthoclase, plagioclase, quartz, 

 hornblende, mica, magnetite, pyrites. In addition, by aid of the micro- 

 scope, numerous crystals of titanite and apatite are seen. 



Orthoclase is the predominating feldspar, and, from the large size of 

 many crystals, as well as from their abundance, gives character to the mass. 

 Many opaque orthoclase crystals measure from 1^ to 2 inches in length, 

 and are remarkable for the well-developed terminal planes, exhibiting forms 

 heretofore entirely unknown in recent volcanic rocks. 



The faces shown will be found in Professor Zirkel's report.^ But singu- 

 larly enough, these rare forms are found again on Steves Ridge, in the 

 Whitehead Peak trachyte region, west of the Park Range, in an undoubted 

 sanidin-trachyte. 



Plagioclase crj'stals occupy a much more subordinate position, are 

 always small, but marked by a resinous lustre. Both quartz and horn- 

 blende play a secondary part. The former occur as clear, colorless grains, 

 and the latter as small, brownish crystals. The mica, which is quite abund- 

 ant, forms well-developed flakes of a brilliant bronze color. The cubes of 

 iron pyrites on a fresh fracture present a bright-yellow color, and appear to 

 be everywhere scattered through the rock-mass. Both the pyrites and the 

 magnetite decompose so readily that the surface of the rock shows frequently 

 a dirty -brown color. In the specimens collected from Parkview Peak, the 

 rock presents a somewhat different appearance, although, as already men- 

 tioned, the microscopical analysis shows the same detailed structure. The 

 latter rock possesses a much lighter color, with a slightly greenish tinge, and 

 a characteristic trachytic texture. All the mineral ingredients found in the 

 rock from the Middle Park Pass occur here, but in different proportions; 

 the individual crystals being smaller, with less of a porphyritic texture, and 

 the bronze mica less prominent, being replaced by dark-green hornblende, 



* Microscopical Petrography, vol. 6, 68. 



