﻿268 NEW YORK STATE MUSEUM 



plentiful. Serpentine, tremolite, diopside, actinolite, occasionally 

 chondrodite, and rarely metalic ores are found. In many cases 

 the limestone passes by transition gradually into a more and more 

 silicious facies until the rock is simply a silicious Fordham gneiss 

 with quartz, mica and feldspar as the essential constituents. There 

 is seldom a sharp break between the two types. Many pieces of 

 apparently simple gneiss will show effervescence of a carbonate 

 constituent with acid. 



The silicious beds of the gneiss series proper immediately associ- 

 ated with the limestone layers are also more silicious or more mi- 

 caceous than the average Fordham. They are essentially micaceous 

 quartzites and mica schists and the rock generally lacks the strong 

 black and white banding that characterizes the common or typical 

 Fordham gneiss of other localities. It is this facies of the gneiss 

 which most closely resembles certain facies of the Manhattan schist, 

 and when the rock is much decayed or badly broken or is ground 

 to pieces by the drill the confusion is still greater. The micaceous 

 variety may readily be mistaken for Manhattan schist and the ac- 

 companying limestone may equally be mistaken for Inwood. 



The occurrence of interbedded limestones of the Fordham series 

 is probably more common than was formerly believed. They are 

 not very often seen on the surface areas of gneiss. Possibly this 

 is largely due to differential weathering and erosion which together 

 tend to obscure those portions of outcrops where such beds may 

 occur. But the type is well known. Mr W. W. Mather in his 

 Geology of the First Geological District [1843] interpreted certain 

 limestones in the Highlands as interbedded in their relation to the 

 gneisses there. Later workers were inclined to disregard his views 

 on this point and there was a marked tendency to place all lime- 

 stone occurrences in one formation. Some of the geological maps 

 have been made in this way. The writer, however, raised the issue 

 again in an article published in 1907 under the title " Structural and 

 Stratigraphic Features of the Basal Gneisses of The Highlands," h 

 N. Y. State Museum Bulletin 107. It is certain that there are inter- 

 bedded limestones with the gneisses in The Highlands. More re- 

 cently, the writer has recognized similar occurrences in the typical 

 Fordham gneisses of The Bronx, New York city. The vicinity 

 of Jerome Park reservoir is the best locality in all southeastern New 

 York to see this interbedded development. The best exposures are 

 at the following places. 



1 In the margin of Jerome Park reservoir at 205th street. 



