76 Iddings and Penfield — Fayalite in Obsidian of Lipari. 



The inflation was not sufficient to produce pumice, as in the 

 case of the obsidian flow at Obsidian Cliff in the Yellowstone 

 National Park.* 



The spherulites and lithophysse that occur within the lava 

 sheets some distance from their upper surface are small. They 

 are very abundant in places and are distributed irregularly, 

 and also in layers. Occasionally, finely spherulitic bands ren- 

 der the rock lithoidal. The obsidian is jet black, but on thin 

 edges it is very transparent, and the light gray spherulites may 

 be seen at some depth within the rock. The hollow spheru- 

 lites in the obsidian stream from the Forgia Yecchia on the 

 east side of the island of Lipari are complex-looking bodies. 

 At first glance they appear to be gray, hollow shells with a 

 rudely spherical kernel at the center. Upon closer examina- 

 tion it is seen that the kernel is a highly crystallized spherulite 

 with distinctly radial structure combined with many small 

 globules. The center of the kernel is dense and gray, but the 

 outer portion consists of a bristling mass of acicular crystals, 

 radiating outward, together with the crystalline pellets already 

 mentioned. The same crystals coat the inside of the surround- 

 ing shell, and in many places are continuous with those of the 

 kernels, and are evidently the same growth. The shell is dense 

 with a sub-vitreous luster, and has a narrow white band parallel 

 to the inner margin. The amount of space between the ker- 

 nel and shell varies considerably. In the smaller spherulites 

 there is very little, and the whole body is clearly one spheru- 

 lite consisting of a central portion with distinctly radial struc- 

 ture, surrounded by concentric shells of slightly different 

 characters. The innermost of these shells is highly crystal- 

 line, white and porous, with minute spaces between the acicular 

 crystals. The next shell is dense, gray and sub-vitreous, and is 

 followed by a narrow white one; the outside, broader shell 

 being dense, gray and sub-vitreous. In the larger spherulites 

 the porous zone has become so porous or open that it forms a 

 cavity between the central part of the spherulite and the outer, 

 denser zones. 



Within the larger cavities the white pellets are recognizable 

 as spherical groups of tridymite, and with them are associated 

 occasional thin tablets of fayalite, which, however, are not 

 found in all of the spherulites. 



A microscopical examination of the spherulites shows that 

 they are beautifully crystallized at the center, the radiating 

 needles being alkali feldspar. Tridymite is also present, in 

 places being clustered in spherical groups of minute crystals. 

 The structure is the same as that observed in some of the 



* Obsidian Cliff, Yellowstone National Park, by J. P. Iddings, Seventh Annual 

 Report of the U. S. Geological Survey, 1888, p. 256. 



