26 Smytk^ Jr. — Pet7'ography of Dikes in Syracuse^ W. T. 



Art. Y1. — Petrography of Recently Discovered Dikes in 

 Syracuse, N. Y. ; with Note on the Presence of Melilite 

 in the Green Street Dike ; by C. H. Smyth, Jr. 



The freshest specimens of the dike rock from Butternut st., 

 Syracuse, ^^iT. Y., are nearly black, but in the average material the 

 color is gray, with a more or less decided greenish tinge. In tex- 

 ture, the rock is distinctly porphyritic, with phenocrysts ranging 

 up to 15°"°^ or more in diameter, scattered somewhat unevenly 

 through a moderately fine and even-grained groundmass. 

 Occasional veinlets and small pockets of calcite appear, but 

 these are exceptional, and the rock as a whole, aside from the 

 '' sheet " material mentioned below, is quite homogeneous. Of 

 the specimens sent to the writer for examination, many appear 

 to the unaided eye quite fresh and unchanged from their 

 original condition ; but they are very deceptive, as in most 

 cases alteration has progressed to a considerable degree. Sev- 

 eral sections cut from sound specimens of good color and with 

 lustrous dark phenocrysts, under the microscope show little 

 but a mass of alteration products, which, however, retain the 

 original texture of the rock very well. In consequence of this 

 alteration the following description of the petrography of the 

 dikes is based upon only a few sections of fresh material, 

 together with such data as are afforded by the altered speci- 

 mens. But the petrography is comparatively simple, and the 

 data at hand are sufficient to show a substantial agreement 

 with the other dike rocks of the vicinity. 



Sections of the least altered material show that the pheno- 

 crysts are olivine, as indicated megascopically. Crystal outline is 

 generally lacking or at most quite imperfect, and the irregular 

 grains show the customary alteration to serpentine along cracks 

 and around the margin. In some grains this alteration is but 

 slight, while in others the serpentine has wholly replaced the 

 olivine. Between the two extremes there is complete grada- 

 tion. Associated with the olivine there is a very little colorless 

 monoclinic pyroxene. It occurs in irregular grains, and shows 

 the usual high extinction angles. Though in very small 

 amount, this pyroxene is a fairly constant constituent of the 

 rock. 



Biotite of a pale brown tint appears in large irregular plates 

 and shreds of small size. The former are primary, but the 

 appearance of the shreds and their relations to the other min- 

 erals suggest that they may be, in part, secondary. Ferofskite 

 is abundant, though varying considerably in quantity in dif- 

 ferent sections. It appears in minute crystals of sharp outline. 

 In some cases, high powers show these crystals to be octahedra. 



