1887. | NEW YORK ACADEMY OF SCIENCES. dd 
micaceous gneiss. The layers are coarser and when contorted 
do not exhibit such fine crumplings as the schist. 
3. Gneiss.—This is a dull gray quartzose rock, occurring in 
thick and thin beds. It is most characteristically shown when 
taken in sections of considerable extent. On a hand specimen 
it appears simply as a structureless quartzose mass. It is often 
interstratified with the schist, giving an alternation of hard and 
soft layers. It contains some feldspar and mica, but quartz is 
the prominent constituent. 
The gneiss in general sometimes shows a tendency to break up 
into large irregular rhombohedra or inclined prisms from cracks 
or cleavage planes whose larger angle is about 105°. These 
peculiar volumes often give a vertical face, a ‘‘step” structure. 
They are well shown at the north end of Tenth avenue and at 
the foot of 50th street, Kast River. 
Hornblende offers a close parallel to the mica, but is very much 
less abundant, There is a hornblende schist which is almost. 
entirely formed of that mineral. It lies in flat crystals, bedded 
parallel to each other, so that it cleaves in thin masses. Indeed, 
in the early days of the city an attempt was made to quarry it as 
slate. Cozzens, p. 15, describes the hornblende rock as slate, 
and the name was not inappropriate. But all attempts to use 
it for economic purposes were failures. Hornblende schist 
graduates into hornblendic gneiss and massive hornblende. 
They all lie interbedded with the gneiss in thick and thin layers, 
but structurally differ little from it. 
The hornblendic rocks exhibit their peculiar property of split- 
ting up from the influence of the weather or water into longitu- 
dinal columns, so that an exposed surface seems like an end view 
of a pile of irregular logs. 
The gneiss also contains many segregated quartz veins, but I 
have not noticed any of considerable size on the island. The 
quartz is ferruginous, vitreous, and milky. 
Granyte.—The western side of the island is largely made up 
of granyte, but it appears everywhere to bea veinstone, either 
filling segregated veins parallel to the bedding or cross fractures. 
Its structure varies from fine compact, to a very coarsely crystal- 
line mixture of feldspar in large cleavage masses, mica in broad 
leaves and quartz as a matrix. The feldspar is generally ortho- 
clase of the pink variety, but plagioclase is by no means rare, and 
often exhibits the twin structure with great distinctness. The 
close-grained granite has been, and is still quarried for building 
stone whenever a street is cut through it, and is much superior 
to the gneiss. 
The segregated veins are exhibited in many places. Along 
the line of Tenth avenue, 48th to 54th streets, there is the 
