180 
I Assembly 
The excavations for the above avenues have rendered Geology an es- 
sential service by exposing the rocks of the island , and exhibiting their 
surfaces, their stratification, their mineral ingredients, and included mi- 
nerals. The strata of the rock, as before stated, (page 178) follows more 
generally the direction of the avenues, which run north 35° east. Of 
about seventy-five observations on this section, to ascertain the strike of 
the strata, more than fifty gave results varying from north 25° east, to 
north 35° east, making the medium strike north 30° east. Two ex- 
tremes were north 45° east, and north 45° west. 
The dip of the strata was taken in eighty-four different places on this 
section. Of these, twenty-nine were vertical, thirty-eight were to the 
west, and eight to the east. Of the thirty-eight which dip westward, 
twenty-four are between 80° west and vertical, seven between 70° west 
and 80° west, and three between 45° west and 70° west. Of the eight 
which dip eastward, six are from 80° east to vertical, and two are 45° 
east. Of the eighty-four observations, fifty-eight were within 10° of 
vertical, and only six gave an eastward dip. The result is, therefore, 
that the medium dip of all the rocks of this section is westward about 
85°. 
The quality of the rock which forms the substratum of this section, 
does not differ much from that of other parts, whether north or south 
of it; therefore, with some few exceptions, a proper description of the 
rock as it occurs here, will answer for the whole. 
As to its components, it contains a large proportion of mica, a small 
proportion of quartz, and still less of feldspar; but generally an abun- 
dance of iron pyrites, (sulphur and iron) in very minute crystals, which 
on exposure, are decomposed. The sulphur and the iron both take 
oxygen from the air, and the result is free sulphuric acid, copperas and 
iron rust. The first two combine in their agency to hasten the disinte- 
gration of the rock, and the third gives it a permanent ferruginous cast. 
In consequence of these ingredients, it is generally fissile, tender, and' 
soon disintegrated on exposure, rendering it unfit for the purposes of 
building. 
In some cases, the rock is so highly charged with pyrites, that on ex- 
posure for a few days in a dry season, it becomes covered with copperas 
in the state of an eflflorescent powder, exhibiting the appearance of 
white frost in an autumnal morning. This phenomenon I have repeat- 
edly seen on the rocks at the Hell-Gate ferry. 
