THE GEOLOGY- OF THE SYRACUSE QUADRANGLE 49 



THE DEWITT DIKE 



In 1894 P. F. Schneider discovered a new occurrence of perido- 

 tite, similar to the Green Street rock, at the Dewitt reservoir. This 

 rock was described by N. H. Darton (35) (36) and J. F. Kemp 

 (35). It was also described by Luther in his Report on Onondaga 

 County (39) and an analysis of the rock was published by H. N. 

 Stokes in Bulletin 148 of the United States Geological Survey. 



DESCRIPTION OF THE SYRACUSE AND DEWITT DIKES 1 



The Syracuse dike is known to extend about seven-eighths of a 

 mile north from Green street ; how much farther is not known 

 owing to the heavy mantle of glacial drift that covers the adjoin- 

 ing area. It is now exposed on Green street in the middle of the 

 street and on the bank on the north side of the street. During the 

 past decade it has been exposed in excavations for sewers along 

 Highland avenue and Farmer street, some of which extending down 

 to depths of 20 feet or more exposed the fresh unweathered rock. 

 From these excavations the dike, or rather system of dikes, is known 

 to extend nearly a mile ; how much farther will not be known until 

 future excavations expose it to view. 



Excavations into the dike mass showed not a single dike but a 

 complex of them in several places more or less parallel dikes vary- 

 ing from 5 to 35 feet thick; some of these have been traced for 

 some distance, but the excavations were too limited to map the 

 separate dikes. It is quite probable that these separate dikes are 

 all united at some point or points below the surface and form 

 parts of a single igneous intrusion that has found its way to or 

 near the surface through a series of cracks. There seems to be 

 no way of determining definitely whether or not these dikes poured 

 any of their contents out over the surface of that time, nor is it 

 possible to tell how great a thickness of rocks has been eroded from 

 the present surface since the dike was intruded, but the bending 

 upward of the shales in the walls of the dikes in several places 

 would indicate, although it does not prove it, that there was not a 



1 This description is based on the papers of the writers referred to above, 

 supplemented by my own observations. I never saw the original exposure 

 on the "Foot Street Road" (now James street) nor that in the Dewitt 

 reservoir, but did see the exposures in the sewer excavations along High- 

 land avenue, Farmer and Butternut streets. T. C. H. 



