Iddings and Penjield — Fayalite in Obsidian of Lipari. 77 



spherulites at Obsidian Cliff, and described in the paper already 

 cited. The outer shells are fibrous, the gray ones being col- 

 ored by a cloudy material, which is light browD in transmitted 

 light, and does not appear to be doubly refracting. The feldspar 

 fibers throughout the spherulite are variously orientated in the 

 zone of their elongation, so that they extinguish light between 

 crossed nicols in groups at different angles to the plane of 

 polarization, and exhibit no definite dark cross. In this respect 

 also the spherulites correspond to those at Obsidian Cliff. 



The chief interest, however, attaches itself to the fayalite 

 crystals in the cavities, which have not been noticed heretofore. 

 They are not abundant, but occur in several localities, having 

 been found by the writer at Forgia Yecchia, and in the obsid- 

 ian stream on Volcano, and having been noted in specimens 

 from Monte della Guardia on the island of Lipari. In the first 

 and second occurrences just mentioned the fayalite is perfectly 

 fresh and transparent. In the third it is more or less altered 

 and opaque. 



The crystals of fayalite found at Forgia Yecchia are very thin 

 plates, the largest being about l mm long, - 5 mm wide and less 

 than 0"03 mm thick. The habit of the crystals is shown in the 

 accompanying figure. The forms which were 

 identified are a, 100, i-l ; 5, 010, i-i ; m, 110, 

 I\ k, 021, 2-i; and e, 111, 1. A slight varia- 

 tion in habit is sometimes caused by the greater 

 development of the e faces. It should be 

 noted that the prism in this occurrence is m, 

 110, I, and not s, 120, as in the fayalite from 

 Obsidian Cliff, Yellowstone Park, and that the 

 basal plane c, 001, is wanting. Yery accurate 

 measurements of course can not be expected 

 from such very minute crystals, but by em- 

 ploying a strong illumination and a low ocular lens, d of 

 "Websky, the following angles were measured on a Fuess goni- 

 ometer, which serve perfectly for the identification of the 

 forms. The calculated angles are derived from the axial ratio 

 established on the fayalite from the Yellowstone National 

 Park.* 





- 



Measured. 



Calculated. 



a ^ e, 



100 a 111 



42° 5' 



42 c 27' 



e /s e, 



111 ~ 111 



95° 38' 



95° 6' 



b*k, 



010 a 021 



41° 2' 



40° 49' 



a y. m, 



100 a 110 



24° 30' 



24° 38' 



m A.m, 



110*110 



130° 35' 



130° 44' 



Cleavage is distinct 'parallel to b, 010. The crystals have a 

 honey-yellow color and show no perceptible pleochroism. In 



*This Journal, III, xxx, 1885, p. 58, also Seventh Annual Report of the U. S. 

 Geological Survey, 1888, p. 271. 



