and its Spherulitio Crystallization. 99 



They average about 7-8""" in diameter. In addition to the 

 regular spherulites various modifications of tliem exist in this 

 place, as may be seen by reference to the plate. Some are 

 composed of only a few blades, or even only two or three, 

 radiating from a center in a star-like group, but with blades 

 the same size and length as those of the more complete spher- 

 ulites. The beauty of tliese white radiate and stellate crystal- 

 lizations suspended in the clear sea-green glass is very striking, 

 and the fact that they are thus suspended and did not fall to 

 the bottom shows that at the time of their formation the molten 

 glass was in too viscous a condition to pertnit of such move- 

 ment, a fact whose bearing upon the question of their origin 

 will be treated later. The bottom of this sheet of glass is 

 composed of a layer of spherulites about 3-4""" in thickness. 

 Looking at the bottom surface itself it appears flat and smooth 

 bat so thickly covered with the radiate crystallizations of 

 these spherulites, seen in half section, that they nearly every- 

 where coalesce, or are contiguous. The appearance recalls 

 surfaces covered with radiate zeolites or frosted window-panes 

 in winter. The upper surface of this layer presents a some- 

 what mossy appearance as the rounded, bladed, surfaces of the 

 spherulites project into the glass. It is seen on the right hand 

 side of figs. B and C in the plate. The white cloudy area near 

 the top of the piece in fig. C is due to the internal reflection 

 of light in the glass and neai'ly conceals a very fine spherulite. 

 The blades composing these spherulites are small at the center 

 where they unite and grow larger gradually as they extend 

 into the glass. They are four sided with apparent right angles 

 and are terminated by an oblique plane, but the crystallization 

 is rough and imperfect. Close inspection of the photograph 

 will show these details. 



Mineral Composition. — The study of the mineral compos- 

 ing these spherulites in thin sections and in powdered gi'ains 

 under the microscope proves that it is artificial diopside. This 

 is seen from the following properties: inclined extinction, 

 c on c measured to a maximum of 39° ; sections having par- 

 allel extinction show the exit of an optic axis on the edge of 

 the field and that the plane of the optic axes lies in the clino- 

 pinacoid parallel to the length of the prism ; by repeatedly 

 measuring the width of the nearly square prisms as they lie in the 

 glass their thickness is also obtained, and the birefringent color 

 yielded by those having the maximum angle of extinction indi- 

 cates that the maximum birefringence is about 0'03. The cross 

 section of the prisms shows them to be nearly square, but they 

 are too minute to show the cleavage parallel to the prismatic 

 sides ; the extinction bisects the angle. These are the proper- 

 ties of diopside and the physical determination was confirmed 



