28 Smyth^ Jr. — Petrography of Dikes in Syracuse^ N, Y. 



occasional grains of garnet. The inclusions are so numerous as 

 sometimes to equal or exceed the dike rock in quantity, and it 

 is probably due to their presence that the rock goes to pieces 

 very rapidly when exposed to the atmosphere. This may be 

 due in part to the chemical effects of the inclusions, but more 

 largely to mechanical influences, the homogeneity of tlie rock 

 being destroyed, while it is filled with fragments, many of 

 which disintegrate easily. The material is most unsatisfactory 

 for study and affords very poor sections. The following 

 description is, for this reason, quite incomplete. 



The garnet appears rather scantily in irregular, rounded 

 grains, sometimes reaching 4 or 5'^™ in diameter. The color is 

 bright red, becoming very pale in thin sections. Around most 

 of the grains there is a very thin shell somewhat distinct from 

 the rest of the rock, but the nature of the material is such that 

 no good section of the shell could be obtained. Apparently 

 mica (perhaps with hornblende) is the chief constituent. In 

 one case the shell consists of decomposition products, through 

 which are scattered great numbers of perofskite crystals. 

 Somewhat analogous to this is the occurrence of a mass some 

 gmin Ijj diameter consisting largely of tiny crystals of perofskite, 

 the whole surrounded by a shell of biotite. 



In every case seen by the writer, the garnet occurs in that 

 part of the rock which contains abundant inclusions, but never 

 in the inclusions themselves. This relation to the inclusions, 

 together with the scantiness and irregular distribution of the 

 garnet, suggests that rather than a normal product of the dike 

 magma like that of the Elliot Co., Ky. peridotite described by 

 Diller"^ and that of many European occurrences, it may result 

 from the fusion of parts of the wall rock in the molten dike, 

 somewhat as in the case of the sapphires of Yogo Gulch, 

 described by Pirsson.f However, conclusive evidence upon 

 this point is lacking. That the garnets are themselves inclu- 

 sions derived from the wall rocks is hardly possible. Were 

 this the case, garnets should be found in some of the inclusions 

 themselves, and even did this happen, it would be needful to 

 prove that they were primary, and not formed by the contact 

 action of the dike rock. Moreover, the only external source 

 of garnets would be in the underlying pre-Cambrian rocks, 

 and these are of sucli a nature that they would be most 

 unlikely to fuse completely away from the garnets, leaving the 

 latter free. 



Embedded in one of the garnets was a bright green grain 

 about 0*5™™ in diameter. When broken and placed under the 

 microscope it shows one cleavage, with which the extinction 

 makes a high angle, while the green color is hardly perceptible. 



* Bull. 38, U. S. G. S. t This Journal (4), iv, pp. 421-433. 



