PYPvITIFEKOUS POKPHYRY— MOSQUITO PORPHYRY. 327 



able from tbe white giouudiuass, and plagioclase is but seldom ideutiflable with the 

 uaked eye. There are uo large ieldspar crystals, as iu the Gray Porphyry. Quartz 

 occurs most frequently in irregular fragments and rarely contains bays of the ground- 

 mass. Biotite appears iu distinct leaves, usually altered to a green chloritic substance. 

 Through a nearly parallel arrangement of its leaves a stratified ai)pearance is pro- 

 duced iu som^ cases. Before disintegration of the rock, the place of the biotite is 

 often occupied by ocher derived from the decomposition of pyrite. The latter mineral 

 is scattered through the whole rock, but concentrated upon fissure planes by secondary 

 processes. Galena appears locally in small quantity, but only ou fissure planes. Some 

 specimens contain irregular fragments of other rocks, chiefly quartzites of the "Weber 

 Grits formation. 



Microscopical — >Jo additional original constituent is shown by the microscope, with 

 tile exception of minute crystals of zircon. Apatite, so seldom wanting in rocks of this 

 class, has not been identified in the Pyritiferous Porphyry. Pyrite takes the place of 

 magnetite and seems to be an original constituent. Its particles are included in quartz 

 and appear in arms of the groundmass, which penetrate or separate quartz grains. 

 It is also seen imbedded iu biotite and is scattered through the groundmass in the 

 manner characteristic of the original ore minerals in similar rocks. Few of the feld- 

 spars are entirely fresh and most of them are replaced by very fine aggregates of 

 muscovite or kaolin. Plagioclase is identifiable in rare cases and was undoubtedly 

 much subordinate to orthoclase iu the fresh rock. In the freshest specimen obtained, 

 chemical analysis showed 4.62 per cent, of potash and 2.91 per cent, of soda. Quartz 

 ai)i)ears in angular grains which are sometimes fractured and show parts of but slightly 

 different oi>tical orientation, separated by thin arms of the groundmass. Fluid inclu- 

 sions are abundant iu many grains, usually with but little fluid, while empty i)ores 

 are also numerous; but none of glass was seen. Biotite is altered to chlorite or allied 

 products, with a separation of yellow needles and tabular crystals, presumably rutile 

 and anatase, respectively. 



The groundmass never reaches the coarseness of grain common in other porphy- 

 ries of the region. It is always very finely and evenly granular, never allowing a dis- 

 tinction of quartz and feldspar. 



MOSQXJITO PORPHYRY. 



This type of quartz-porphyry, found in several distinct bodies and exhibiting in 

 all a marked uniformity iu structure and composition, has been named from its princi- 

 pal observed occurrence in the North I\Iosquito amphitheater [98]. All the bodies are 

 dikes ill the Archean, and besides the locality mentioned the rock was seen upon the 

 north wall of Mount Lincoln [97] and in Cameron amphitheater [96], in the latter case 

 penetrating sedimentary beds. 



It is a light gray rock of fine grain, whose most prominent constituent is quartz 

 in clear, irregular grains, which seldom exceed 0.5'"" in diameter. Other recognizable 

 elements are biotite in small leaves, not abundant, and minute feldspars, which can 

 scarcely be distinguished from the light groundmass. A brilliant, black ore in small 

 specks is abundant. Glistening hexagoual prisms of what tbe microscope proves to be 

 apatite are often seen, upon close examiuation. 



