ZIEGLER, THE RAVENSWOOD GRANODIORITE 7 



Recasting into the component minerals we obtain 



Per cent 

 Quartz 18.48 



Orthoclase 8.34 



Albite 44.02 



Anorthite 13.90 



Grossularite 2.25 



Almandite 3.48 



Hornblende and Augite 7.27 



Biotite 1.05 



Corundum 91 3 



Magnetite 70 



Probable Extent of the Ravenswood Granodiorite as Disclosed 



by Borings 



Aside from the actual outcrops of the granodiorite so far discussed, 

 records of it are found in borings undertaken in the drift-covered areas. 

 Thus, north of the exposures and slightly southeast of Lawrence Point 

 the Consolidated Gas Company of New York while drilling for water put 

 down an eighteen-hundred foot borehole. The entire core is the typical 

 Ravenswood rock. Again, south of the actual exposures, a series of bor- 

 ings 4 is now available from the corner of Myrtle Avenue and Gold Street. 

 Brooklyn, across the East River into Manhattan, as far as Hester and 

 Allen Streets. Several other drill records are also known as depicted on 

 the map. In all of these, the core is a massive gray rock, in all respects 

 like the exposed Ravenswood. It has the same appearance, texture and 

 mineral composition, and it seems to be a part of the same igneous mass. 

 Whether the rock is actually continuous so as to include all these areas, 

 cannot be said with certainty, but the conclusion seems probable, and the 

 corresponding extent has been so indicated on the accompanying map. 



Origin of the Garnet 



Some of the cores brought up on the above line and located near the 

 intersection of John and Bridge Streets show a highly garnetiferous rock, 

 reddish-brown in color. The rock is finely granitoid. Under the micro- 

 scope, the following minerals were found : garnet and quartz as essential, 

 also some feldspar, which varied extremely in different specimens, and 



3 The corundum molecule in this and the following analyses is probably to a great 

 extent contained in the biotite. 



* The borings and cores here referred to, samples from which were made available for 

 this study, belong to the series of explorations made by the New York City Board of 

 Water Supply along the proposed Hue of the distribution conduit for Catskill water. 



