Smyth ^ Jr. — Petrography of Dikes in Syracuse^ JY. Y. 27 



and this is probably, as would be expected, the form of all of 

 thera. They are honey yellow, and decidedly translucent, so 

 much so indeed as to show this property clearly with low 

 powers. In this respect, the perofskite differs from that of the 

 De Witt dike, in which Prof. Kemp^ perceived the translucent 

 character only with high powers. It would be difficult to find 

 better microscopic specimens of perofskite than are shown in 

 the rock here described. 



Magnetite is another primary mineral, occurring in the usual 

 small crystals and irregular grains. But it is far less abundant 

 than is often the case in rocks of this general type. In some 

 sections there is a small quantity of a granular, isotropic min- 

 eral with high index of refraction. While this may be garnet, 

 no exact determination of the species could be made. In 

 addition to the foregoing minerals there is present, even in the 

 freshest specimens, a considerable amount of undeterminable 

 alteration products mingled with the serpentine, carbonates, 

 secondary magnetite, mica shreds and the minor primary 

 constituents to make up the groundmass. While it is possible 

 that there may have been glass in the unchanged rock, none 

 is now recognizable. 



Much of the foregoing description of the freshest parts of 

 the dike applies equally well to the more altered parts. In 

 these, the olivine has been almost wholly changed to serpentine, 

 which is greenish or yellowish and shows the characteristic net 

 texture to a marked degree. The double refraction of the 

 serpentine is very low, and in the thinnest parts of sections, 

 and often elsewhere as well, it is sensibly isotropic. Different 

 specimens show great diversity in the amount of secondary 

 magnetite in the serpentine. In some cases the former min- 

 eral is scantily sprinkled through the latter, while in other 

 cases the serpentine is rendered nearly opaque by the powdery 

 magnetite. The primary magnetite and perofskite are not 

 affected by alteration, the latter in particular retaining all of 

 its characteristic properties and showing in the sections of 

 thoroughly altered specimens perhaps even better than in the 

 fresher rock. All of the foreo:oino^ chans^es belono; to the 

 process of alteration rather than weathering, and do not affect 

 the integrity of the dike as a whole, while modifying its com- 

 position, both mineralogical and chemical, to an unknown 

 depth. Weathering on the other hand, though superficial, 

 destroys the dike, as far as it goes, resulting in thorough 

 disintegration. 



The rock first called a sheet, but now regarded as probably a 

 distinct dike, differs from that described above chiefly in con- 

 taining abundant inclusions of the wall rocks together with 



* This Journal (3), xlix, p. 459. 



