lio new york State museum 



down the feldspar into granular aggregates, it seldom shows any 

 well developed schistosity. Near the ore the gneiss is seamed 

 through and through by pegmatite of lighter color and is also 

 penetrated by a fine-grained granite like that found on Birch hill. 

 Bands of somewhat darker appearance are not infrequently inter- 

 calated in the gneiss. They are apt to be more hornblendic than 

 the surrounding rock and are probably to be interpreted as masses 

 of the hornblende schist, which is described on a following page, 

 that have been penetrated by, and, to a greater or less extent, 

 incorporated with the augite gneiss. The origin of the latter rock 

 is believed to be igneous; in physical character and mineralogy 

 there is a close accord with the plutonic rocks such' as are found 

 among the basal formations of the Adirondacks. That it belongs 

 probably to the older series of this class is indicated by its cata- 

 clastic texture along with the presence of similar intrusives in the 

 vicinity that have been but little affected by dynamic agencies. 

 Though the chemical constitution of the gneiss has not been 

 determined by analysis, there would seem to be little doubt that 

 the relative proportions of the constituents agree with those of the 

 acid igneous class, varying from syenite to a low-quartz granite. 



This gneiss underlies the greater part of the area about Lyon 

 Mountain. It constitutes most of the high ridge east of the ore 

 bodies as well as the projecting spurs, and probably extends 

 beneath the drift-covered valley to the north and west. It forms 

 the walls in most of the mine openings and is invariably closely 

 associated with the ore. 



Granitic gneisses. On the top of Lyon mountain a coarse quartz- 

 feldspar rock of slightly gneissoid appearance is exposed over an 

 area that can not be accurately delimited, though it is probably 

 small. Judging from surface indications it extends a few hundred 

 feet down the slopes, which are thickly strewn with its boulders, 

 but no contacts were found. The rock has the composition of an 

 acid granite, with which it is allied so closely in texture and field 

 structures as well, that little doubt of its igneous nature can exist. 

 Its mineralogy is simple, feldspar and quartz forming almost the 

 whole mass. The latter mineral occurs in flattened lenses and 

 spindles which have a common orientation and give the somewhat 

 indefinite gneissoid appearance observable on weathered surfaces. 

 Microcline predominates over orthoclase, the two feldspars repre- 

 sented. Both show commonly a microperthitic habit. Of the 

 ferromagnesian minerals there are a few scattered grains of green 

 augite and small shreds of biotite. In outcrop the rock exhibits a 



