ANNALS NEW YORK ACADEMY OF SCIEN8ES 



Surface Distribution of the Ravenswood Granodiorite 



The western part of Long Island is covered by drift, so that exposures 

 of rock are rare, except along the shore of the East Eiver and about the 

 BlackwelPs Island bridge. Here is found a series of scattering outcrops, 

 beginning near the corner of Boulevard and Potter Avenues and recurring 

 at intervals as far south as Fifth Street. North and south of the above 

 areas, drift masks everything, and the only clue to the underlying forma- 

 tions is afforded by a series of bore-holes which will be described later. In 

 the New York City folio, most of the exposures of this rock are mapped 

 as separate acidic intrusives in the Fordham, but the idea that these repre- 

 sented one large igneous body was not expressed. Since there are no 

 traces of Fordham gneiss found between the above exposures, and since in 

 a number of cases granodiorite was found between some of the outcrops in 

 the excavations for the foundations of buildings, it seems to the writer 

 that we may safely map this whole area as granodiorite. Thus we cut off 

 at once an area of at least a square mile from the old Fordham formation 

 and add it to the series of igneous intrusives mentioned above. 



Petrographic Description 



The Ravenswood granodiorite is a light to dark gray rock, varying from 

 granitoid to gneissic in texture and ranging from a hornblende-biotite- 

 granite to a diorite in composition. Garnet is present in nearly all speci- 

 mens and in some cases is very abundant. For the purpose of establishing 

 a type composition for the rock, the following table has been prepared to 

 show the minerals present in a number of slides. A system of notation 

 devised by C. P. Berkey, in which X denotes an essential, Y an accessory 

 and Z a secondary mineral, is used. 



