PYKITIFEROUS POEPHYEY MOSQUITO POEPHYEY. 327 



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

 naked eye. There are no large feldspar crystals, as in the Gray Porphyry. Quartz 

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

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

 Through a nearly parallel arrangement of its leaves a stratified appearance is pro- 

 duced in some 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 on fissure planes. Some 

 specimens contain irregular fragments of other rocks, chiefly qnartzites of the Weber 

 Grits formation. 



Microscopical No additional origiual constituent is shown by the microscope, with 

 the 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 iu quartz 

 and appear iu arms of the grouudmass, which penetrate or separate quartz grains. 

 It is also seen imbedded iu biotite and is scattered through the groundinass iu 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 iu rare cases and was undoubtedly 

 much subordinate to orthoclase in 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 

 appears iu angular grains which are sometimes fractured and show parts of but slightly 

 difterent optical orientation, separated by thin arms of the groundmass. Fluid inclu- 

 sions are abundant in many grains, usually with but little fluid, while empty pores 

 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 iu other porphy- 

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

 tinction of quartz and feldspar. 



MOSQUITO POEPHYEY. 



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

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

 pal observed occurrence in the North Mosquito amphitheater [98]. All the bodies are 

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

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

 penetrating sedimentary beds. 



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

 iu clear, irregular grains, which seldom exceed 0.5 cm iu 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 hexagonal prisms of what the microscope proves to be 

 apatite are often seen, upon close examiuation. 



