50 • NEW YORK STATE MUSEUM 



great thickness of rocks above the present surface. The dike rock 

 in some places appeared to have spread out like a sill between, the 

 layers of shale. The excavations were too limited fully to establish 

 this. 



Fig. 4 Section of Green street dike along excavation made by electric 

 light company, November 24, 1905. Sh = shale P = peridotite 



Both the Green street and the Dewitt dike occur in the lower 

 part of the Camillus shales which here contains intercalary vermicu- 

 lar limestone. The limestone and the shales show the effect of heat 

 at the contact to a very limited extent, indicating that the tempera- 

 ture of the intrusive rock was not excessive nor the high tempera- 

 ture long continued, another evidence that the rocks at the present 

 surface were not deeply buried at the time of the intrusion. 



The intrusion at the Dewitt reservoir and those in the city of 

 Syracuse at Green street, Highland avenue and Butternut street, 

 and at Griffith street are all larger than the dikes at Manheim, Ithaca, 

 Ludlowville and Clintonville. 



Petrography. As already observed, Williams (14) was the first 

 to prove the igneous or intrusive origin of the Syracuse dikes as 

 previous to his examination they were considered to be metamor- 

 phosed sedimentary rocks. Smyth was the first to make a micro- 

 scopic examination of the Butternut street exposure which he deter- 

 mined to be a porphyritic peridotite with phenocrysts of olivine. 

 With the olivine he found some colorless pyroxene, probably augite. 

 Biotite of a pale brown tint occurs in large irregular plates and 

 shreds of small size ; the latter appears to be secondary. Perofskite 

 is abundant in minute crystals of sharp outline, honey-yellow and 

 translucent. There is some magnetite and a granular mineral with 

 a high index of refraction, which is probably a garnet. The inclu- 

 sions are so numerous in some places, he says, as to equal or even 

 exceed the dike rock in quantity. *' The garnet appears rather 

 scantily in irregular rounded grains 4 millimeters to 5 millimeters 

 in diameter. The color k, bright red." Smyth's statement regarding 

 the size and number of the garnets was evidently based on the few 

 specimens of rock which he had, as at the exposure there were scores 

 of these garnets much larger than he describes. 



