﻿GEOLOGY OF THE NEW YORK CITY AQUEDUCT 1 85 



garnet-bearing quartz-mica schist varying from rather fine grain 

 and semigranular appearance to a very coarse and strongly foliated 

 structure. This part of the formation undoubtedly represents re- 

 crystallized or metamorphosed sediments. But associated with this 

 fades there is a more dense black hornblende schist that, not only 

 here but at many other places, is thought to represent igneous in- 

 trusions that have been metamorphosed together with sediments of 

 various types, until both have lost almoist all of their original char- 

 acters. The hornblendic schist type is not so extensive as the other, 

 the mica schist, but it is more compact and here as usual is in the 

 better condition. 



Pegmatite stringers occur abundantly, especially in the mica schist 

 varieties. They are of no great consequence, however, as a factor 

 in this study. They originated in the aqueo-igneous activity in- 

 volved in the recrystallization of the rock when it was worked over 

 into a schist. 



Beneath this Manhattan schist formation lies the Inwood lime- 

 stone, a bed probably at least 700 feet thick. But at this point how 

 deep it lies and at what depth it would be penetrated nobody can 

 tell. None of the drills have touched it. Beneath the limestone in 

 turn lies the granitic and banded gneisses belonging to the Fordham 

 gneiss series, the lowest and oldest of the region. 



Along the Croton river nothing but Manhattan schist is to be seen 

 at the surface for more than a mile above and below the proposed 

 crossing. The same thing is true for an equal distance on opposite 

 sides from the river at this locality. 



But the structure is folded and the normal northeast-southwest 

 trend of the folds crosses the river, every arch or anticline tending 

 to bring the limestone and gneiss nearer to the surface. One of 

 these folds does expose the limestone and gneiss in a strip extend- 

 ing from the Hudson river northeastward for two thirds of the 

 distance to the old Croton dam. But before reaching the Croton 

 valley this fold pitches down toward the northeast beneath the Man- 

 hattan schist and passes under the present lake (or reservoir) in 

 that relation, not reaching the surface again for a distance of about 

 6 miles. At least one more fold is known to behave in a similar 

 manner as it reaches the Croton. 



These facts make it certain that there is limestone beneath the 

 schist in the vicinity of the crossing, and that it comes nearer to 

 the surface in that vicinity than at some other places. 



South of the Croton there are several small cross faults run- 



