1893. ] NEW YORK ACADEMY OF SCIENCES. 207 
in grains and crystals ; but most abundantly by ilmenite. This 
mineral is seen in nearly every section, often in grains of con- 
siderable size. In some cases it is perfectly fresh, in others the 
cleavage lines and boundaries are marked by white or yellowish 
leucoxene. 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 structure 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 thé larger number of sections 
show the quartz and feldspar in rather large masses surrounded 
by the finer mosaic of fragments. A resemblance to porphyritic 
structure is sometims produced in 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, differing 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 penetrate 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 
eneiss 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 apatite, show crystal outlines. 
The feldspars are quite fresh, but show decided mechanical 
