54 TRANSACTIONS OF THE [Nov. 14, 
and gravel, the deposit of a swift current, and therefore of a 
shallow one. ‘The rock must have been 50 feet nearer the water 
level at least, during the deposition, then to have sunk until 175 
feet of sedimentary strata rested onit, then to have been elevated 
until 100 feet and more projected above the water. If the propo- 
sition that the surface is now sinking again (see above under 
alluvium) be true, the island must be beginning a second great 
cycle of depression, to be followed, perhaps, by another eleva- 
tion. The conclusion is irresistible that this island, and nat- 
urally the neighboring country, has undergone marked changes 
since the glacial epoch, and that during, or immediately after 
that epoch, it stood higher than it does now. 
‘The sections on the map across the drift area are generalized 
from the data given above. 
The division called *‘ Gneiss” above will be taken up in detail 
under the headings, ‘‘Gneiss,” ‘‘ Granyte,” and ‘ Less Impor- 
tant Rocks.” 
Gneiss.—The gneiss formation is made up of a series of rocks 
that vary from mica or hornblende schist through all intermediate 
grades or varying combinations of the component minerals to a 
hard, heavy-bedded quartzose gneiss. ‘They are all more or less 
perfectly stratified and exhibit the evidence of bedding most 
completely, but are cut across by granite veins and hold granite 
interbedded with them. 
1. Mica Schist.—This rock consists almost entirely of a mica, 
either muscovite or biotite, whose crystals are held together by 
acement of quartz. It is stratified, of small coherence and dis- 
integrates easily. The mica crystals lie parallel to the bedding, 
their easy cleavage being the weak point of the rock. ‘The ex- 
cessive mica is what ruins the rock for building purposes, as it 
makes it impossible to break it across the bedding with any cer- 
tainty. The schist is the weak point of the rocks for another 
reason. Being of more open structure, it is the most accessible 
place of attack for disintegrating water, and, where weathering 
has advanced to any extent, the schist has been the first and 
most extensive sufferer. Where the strata are crumpled, the 
schist has often been forced into fine wrinkles or waves whose 
amplitude is as small asaninch. It is more abundant on the 
east side of the island than the west. 
In the district between Fifth and Fourth avenues, 49th and 
57th streets, kyanite is found in considerable quantities replacing 
the mica and forming a kyanite schist, but it is of no great im- 
portance or extent. 
2. Micaceous Gneiss.—As the proportion of mica decreases 
the rock becomes harder and more coherent, graduating into 
