1898.] NATURAL SCIENCES OF PHILADELPHIA. 91 



ilyte were found. On a later visit I observed a slab of about tbree 

 feet in length and about six inches tbick imbedded in a bank of 

 debris of similar material; many indications of the volcanic rock 

 being scattered about. The rock is jet black, dull, very fine grained, 

 with distinct conchoidal fracture, the edges becoming very sharp ; 

 the hardness is a little less than quartz. The per centage of silica 

 was 57.1. Under the microscope was observed a microperlitic 

 structure. All attempts to secure a photograph of this structure 

 failed. 



A second variety of tachilyte was found, but not so abundantly as 

 the first. It is also deep black and very fine grained, very thin 

 white streaks or lines being irregularly distributed through the 

 mass. Under the microscope, the thin section showed again that 

 the essential material consists of devitrified glass darkened by an 

 extremely fine powder of what seemed to be magnetite. The white 

 lines, suggestive of crystals, appeared when magnified as elongated 

 globular forms filled with a partially crystalline substance of indis- 

 tinct crystalline forms. The globules consist of subcrystalline mate- 

 rial, filling what was originally gas pores. A section showed also 

 the fluid structure. With very strong, light, numerous crystallites 

 were observed. 



A third variety of devitrified glass has a yellow-gray color. This 

 tachilyte is very hard and so peculiarly splintery, sharp edged and 

 curved, that no fragment could be secured with the hammer from 

 which to grind a section, the lapidary having to cut the plates for 

 the purpose. The mineral is very fine grained and even, some 

 specimens having a yellow edge apparently of the same hardness as 

 the general mass. Where water had affected the material there is 

 a thin rusty coating. 



With ordinary light, a thin section shows beneath the microscope, 

 that the mass of rock is made up of largely predominating fine 

 ash-like granules interspersed with minute fragments of crystalline 

 material, the whole cemented together by devitrified glass. 1 



1 About six years ago, I found a specimen of rock near Rockhill Station, in 

 Bucks County, and determined it under the name of Felsite. It seems to me, 

 that Felsite, Eurite, Petrosilex and Tachilyte are all the same in composition 

 structure and origin. All these rocks seem to belong to the glassy lavas, and 

 are, therefore, in all probability, of volcanic origin. Since the inner structure 

 and mineral composition should decide the name of a rock, it is thought best 

 to call this, and all three varieties, Tachilyte, because the principal mass is 

 devitrified obsidian, as has already been suggested by several British petro- 

 graphers, and, since Breithaupt's name seems to have the priority, it should, 

 I think, be retained by geologists. 



