10(5 



/.. V. Pirsson — Artificial Lava-Flow 



index of refraction than tire sflass. While everywhere freel_y 

 sprinkled through the field of view, as the stars appear at 

 night in the sky, they cannot be said to swarm in the sense 

 that, with reference to their own diameters, they closely 

 approach one another. They exert no perceptible action on 

 polarized light. To the absorption of light caused by the 

 presence of these minute bodies, as with some natural obsid- 

 ians, the black color of this variety of the Kane glass is 

 ascribed. 



Partly Crystallized Obsidian. — In one place there is a 

 layer of the obsidian about 3 inches thick, which has partly 

 ci'ystallized. The black glass described above makes a rather 

 clean and sharp contact with it. It is not now known whether 

 this layer was above or below the pure glass one, but as it con- 

 tains impurities on the side opposite the glass, and also on 

 general principles, it is assumed as the bottom part of the 

 obsidian. In the specimen it has a stony, not glassy, appear- 

 ance ; is of a very dark to blackish gray color, and is seen to 

 be composed of a sort of felt of innumerable small slender- 

 bladed crystals 2-4""" long, which have a tendency to be 

 arranged parallel to the extension of the layer. It has some- 

 thing of a rouo;h resemblance to some hornblende schists. 

 There is no schistosity in the fracture 

 however, which is rough and hackly. 

 The thin section under tlie microscope is 

 a very interesting one. It is composed 

 of a pale brown glass, filled with beau- 

 tiful crystallizations of the mineral 

 wollastonite. While in general the 

 mineral is developed as usual in long col- 

 umnar forms, or plates parallel to the 

 axis of symmetry, its growth has also 

 been in such aggregates of sheaf-like, 

 rosette-like, or feathery forms, that ac- 

 cording to the way these are cut differ- 

 ent effects are produced. The simplest 

 case is shown in fig. 4, which gives a 

 section parallel to h (010), across one of 

 the bladed crystals. The faces in the 

 zone of a (100) a g (001) are very 

 sharply developed, as may be seen from the following table 

 of angles measured against the cross-hairs : 



/3, a (1^00) ^ c(OOl) 

 a' (100) ^ t (iOl)' 

 a (100) /s y(lOl) 

 a (100) ^ a (102) 



Fig. 4. 



Fig. 4. Wollastonite, 

 section parallel to b (010). 



Meas. 



Calc. 



84° 



84° 35' 



50° .30' 



50° 26' 



42° 30' 



44° 33' 



70° 



69° 55' 



