1893.] NEW YORK ACADEMY OF SCIENCES. 207 



in grains and crystals ; but most abundantly b}' ilmenite. This 

 mineral is seen in neai'ly ever)' section, often in grains of con- 

 siderable size. In some cases it is perfectly frcsli, in others the 

 cleavage lines and boundaries are marked by white or yellowish 

 Jeucoxene. When this alteration has reached its limit, there is 

 left only the leucoxene, either in an irregular mass or forming 

 a network with the angles of the cleavage of ilmenite. Zircon 

 occurs in irregular fragments and in stout prisms. 



In structui'e the gneiss varies from a coarse holo-crystalline 

 granular, or granitic, to a fine mosaic made up of fragments of 

 the constituent minerals. Between these two extremes there is 

 every possible stage. By far the larger number of sections 

 show the quartz and feldspar in rather large masses surrounded 

 by the finer mosaic of fragments. A resemblance to porpliyritic 

 structure is sometimsi:)roducedin this way, particularly when, as 

 occasionally happens, there has been considerable recrystalliza- 

 tion of the finer fragments. 



Variations from the normal gneiss are, as already stated, quite 

 abundant, but can be mentioned only briefly. A garnetiferous 

 gneiss was seen at several points, difiering from the ordinary 

 gneiss chiefly in containing large quantities of garnet. This 

 mineral forms irregular masses of considerable size. Under 

 the microscope it has a decided pink color, and is quite fresh, 

 showing only a slight development of chlorite along cracks. 

 This variety of gneiss often contains an abundance of sillimanite 

 in long slender crystals. The mineral is not evenly distributed 

 through the rock, but is grouped in nests, in which the indi- 

 vidual crystals are often roughly parallel. "While these nests 

 may penetrate quartz, there is a marked tendency for the silli- 

 manite to occur in the garnet. Sometimes only the outer 

 portion of the latter mineral contains sillimanite, but often the 

 slender prisms j^enetrate the entire mass of garnet. 



The black bands and lenses that are so conspicuous in the 

 light colored gneiss are commonly composed of the constituents 

 of the ordinary rock, with the dark minerals in unusually large 

 quantities. They are sometimes hornblendic and sometimes 

 micaceous, while in some instances ilmenite is an important 

 constituent. One such dark band occurring in garnetiferous 

 gneiss is quite different from the others examined. It is a 

 nearly black, micaceous, distinctly schistose rock, of medium 

 grain. Sections show a holocrystalline aggregate of plagioclase, 

 a little orthoclase, monoclinic pyroxene, and hornblende ; with 

 apatite, zircon and pyrite as minor constituents. None of these 

 minerals, except the aj^atite, show crystal outlines. 



The feldspars are quite fresh, but show decided mechanical 



