Adirondack magnetic iron ores 169 



Gneiss series. The gneisses are the most widespread of the 

 rocks represented in this region. With the exception of the 

 numerous but comparatively limited exposures of dike rocks which 

 are hereafter described, they comprise practically all of the crystal- 

 lines adjacent to the ore bodies and occupy as well most of the 

 surrounding country. The occurrence of gabbro and augite 

 syenite has been noted at Rand hill, east of Dannemora mountain, 

 but so far as observed they do not appear anywhere in the imme- 

 diate vicinity of Lyon Mountain. 



In this series is included a complex of rocks differing in com- 

 position and physical characters. The study of their field rela- 

 tionships is attended with much difficulty owing to the heavy 

 mantle of drift over the area affording limited opportunity for 

 observation, and to the variations in the rocks from, place to place. 

 Practically all of the numerous specimens taken from typical 

 exposures in the vicinity of the ore bodies may be classed, how- 

 ever, in the following groups. 



Augite gneiss. This is a reddish or grayish granular rock char- 

 acterized by the presence of augite. It is mentioned by Cushing 

 as one of the predominant types in the group of gneisses designated 

 as the Saranac formation. 



In its prevailing development it consists essentially of feldspar, 

 quartz and augite, with subordinate hornblende, titanite, mag- 

 netite and apatite. The augite is an emerald-green variety and 

 is always abundant, sometimes composing as much as 20 per cent 

 of the rock mass. The feldspar is mostly orthoclase, but small 

 amounts of plagioclase (oligoclase) may be present. The ortho- 

 clase shows a microperthitic intergrowth with albite. The quartz 

 is a fluctuating constituent, though the relative quantity is below 

 rather than above the proportions found in typical granites. The 

 greenish, strongly pleochroic hornblende occurs in skeletal or very 

 irregularly bounded anhedra and may be in part derived from the 

 augite with which it is intimately associated. Of the other com- 

 ponents titanite alone has importance. Most specimens exhibit 

 this mineral abundantly distributed in the form of rounded grains 

 varying from light yellow to reddish brown in color. It also 

 occurs as rims surrounding the magnetite. In some specimens 

 taken from the walls of the ore bodies, the titanite constitutes 

 fully five per cent of the rock mass. 



The field appearance of the gneiss is usually massive, with but 

 faint tendency toward a parallel grouping of the constituents. 

 Though it has undergone more or less crushing which has broken 



