128 NEW YORK STATE MUSEUM 



rangle. The I.osee gneiss is much more obscure and uncertain as 

 to its equivalents and the Byram gneiss, although made to include 

 a very wide range of expressions, as used in New Jersey, is probably 

 in its simplest form a rough equivalent of the Storm King granite 

 of the West Point area and the syenite of the Adirondacks. 



In the West Point quadrangle it seemed at first preferable to 

 assign the basic intrusives resembling the Pochuck to an intermediate 

 position between the Canada Hill granite invasion and the Reser- 

 voir granite rather than to a period older than the Canada Hill. 

 But later study favors the belief that these basic members have 

 different age relations and that the principal basic type to which 

 the name Pochuck gneiss ought to be attached is older than any of 

 the important granites. The best representative is the occurrence 

 referred to as the Peekskill diorite gneiss. In such case it may be 

 that the dioritic-pegmatites and associated magnetites are connected 

 with this member and are older than Canada Hill age. 



Comparison with the Nezv York City area. There is no doubt 

 but that certain facies of the Fordham gneiss of the New York City 

 district represent the Grenville. There is the same sedimentary 

 origin evident in some of the beds, the clearest being numerous 

 limestone layers. Most of them are not large, but they occur in 

 such relation as to make their origin clear. The associated clastic 

 siliceous beds are much more fully transformed and obscure than 

 the limestones because they yielded much better to igneous injection 

 and impregnation. 



Examination of a collection of typical material from the Ravens- 

 wood grano-diorite of New^ York City (Brooklyn) shows resem- 

 blance in a few of the slides to the regular Storm King type of 

 granite. The whole series with its wide range of mineral make-up 

 is believed to be more consistent with the range said to be exhibited 

 by the .syenites of the Adirondacks. The Ravenswood agrees with 

 both in one respect, that is, its apparent late appearance in the series 

 of intrusives. In New York City it furnishes pegmatites which cut 

 both the Inwood and the Manhattan and is itself a very vigorous 

 invasion unit, developing elaborate injection and impregnation and 

 syntectic effects. Perhaps to this latter habit is in large part due the 

 great variety of composition shown in New York City although 

 some of it may be due to actual differentiation. 



Comparison zvith the Philadelphia area. The Baltimore gneiss 

 of the Philadelphia area is regarded as the equivalent of the oldest 

 gneisses of the West Point cjuadrangle and the Fordham gneiss of 



