520 
GEOLOGY OF THE FIRST DISTRICT. 
“ The elevated points of this section of the island vary from seventy to one hundred and 
twenty feet above tide-water mark, and will, when graded, give at a rough estimate a medium 
level of about forty feet above the contiguous waters. The valleys, however, are often deep, 
and the hills precipitous, rendering this section rough and broken, 
“ The Third, Fourth and Eighth avenues have been cut through until they reach the Har¬ 
lem river ; the first two terminate at Harlem, the other at M'Comb’s dam ; Fifth and Seventh 
avenues terminate at Twentyfirst-street; Sixth avenue extends to the Bloomingdale road at 
Thirty second-street; Tenth avenue to the same road at Seventieth-street, in the village of 
Bloomingdale ; and Ninth avenue terminates at Fortysecond-street. 
“ The excavations for the above avenues have rendered geology an essential service by 
exposing the rocks of the island, and exhibiting their surfaces, their stratification, their mineral 
ingredients, and included minerals. The strata of the rock, as before stated (page 519), fol¬ 
low more generally the direction of the avenues, which run N. 35° E. Of about seventy-five 
observations on this section to ascertain the strike of the strata, more than fifty gave results 
varying from N. 25° E. to N. 35° E., making the medium strike N. 30° E, Two extremes 
were N. 45° E. and N. 45° W. 
“ 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 east¬ 
ward, six are from 80° east to vertical, and two are 45° east. Of the eighty-four observa¬ 
tions, fifty-eight were within ten degrees 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 
eighty-five degrees. 
“ 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 felspar ; but generally an abundance 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 disintegration of the rock, and the third gives it a 
permanent ferruginous cast. In consequence of these ingredients, it is generally fissile, ten¬ 
der, 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 exposure for a few days in a 
dry season, it becomes covered with copperas in the state of an efflorescent powder, exhibit¬ 
ing the appearance of white frost in an autumnal morning. This phenomenon I have repeat¬ 
edly seen on the rocks at the Hellgate ferry. Besides the regular ingredients of gneiss, it 
occasionally happens that the mica is replaced, either wholly or in part, by hornblende; in 
which case its color is rendered darker, it is more compact in texture, and columnar in its 
