194 ANNALS NEW YORK ACADEMY OF SCIENCES 



Introduction 



The Manhattan schist is the uppermost or youngest of the three crys- 

 talline metamorphic formations which constitute the bedrock over the 

 southeastern portion of New York State. The other two are the Inwood 

 limestone and Fordliam gneiss. These three formations are well ex- 

 posed at numerous localities in New York, Westchester and the southern 

 portions of Putnam Counties. The overlying mantle of Glacial drift, 

 seldom very thick, has been removed over many portions of the area by 

 erosion and in other places was never laid down. To the north, in Put- 

 nam and Dutchess Counties, the upper two formations, the Inwood lime- 

 stone and the Manhattan schist, are not present, since a belt of gneisses 

 constituting the Highlands of the Hudson intervenes. These gneisses 

 are probably the equivalent of at least a portion of the Fordham gneiss of 

 the region to the south. The overlying formations have here been re- 

 moved b}^ erosion. North of this belt a quartzite appears resting uncon- 

 formably upon the gneiss. It is followed by a limestone and a slate. 



The oldest of these formations exposed in southeastern New York 

 State, the Fordham, is a black and gray banded gneiss made up largely of 

 quartz, feldspar and biotite, with occasional grains of zircon and a very 

 little apatite and titanite. Hornblende is occasionally abundant. Garnet 

 is rare. The feldspar consists mostly of microcline and orthoclase, to- 

 gether with some albite-oligoclase. The rock shows a typical gneissoid 

 structure, the alternate light and dark bands, which rarely exceed one 

 or two inches in thickness, being due to the concentration of the mica 

 along certain bands. A few thin beds of highly crystalline limestone are 

 found associated with the gneiss. Dr. Charles P. Berkey^ was the first to 

 call attention to the fact that these are an integral part of the formation. 

 The presence of interbedded limestone indicates the sedimentary origin 

 of at least a portion of this formation. Dr. Berkey has shown that a fur- 

 ther subdivision of these basal gneisses is impracticable. He correlates 

 them with the Grenville series of the Adirondacks and Canada, which are 

 known to be of pre-Cambrian age. Associated with them are a large 

 number of igneous intrusive masses whose composition varies from that 

 of granite to that of diorite. Most of these intrusives have also been 

 subjected to metamorphic agencies so that they now appear as gneisses. 



A quartzite occasionally appears in the upper portions of the Fordham 

 gneiss to whicli the name Lowerre has been given, since it is from that 



^"Structural and stratigrapliic features of the l)asal siiieisses of the Ilif^hhinds." N. Y. 

 State Mus. BuU. 107, pp. .*',(>l-r{71. 11)07. 



