and its Spherulitic Crystallization. 113 



ferent refractive indices, and thus becoming visible by tlieir 

 action on light, in the same manner that heated air-currents 

 are visible above a source of heat, or differing currents of 

 salt laden solutions are seen in a liquid. These streaks are 

 small and tine and are parallel to one another, and to the 

 chains of spherulites, in the direction of flow. Since the 

 refractive index varies in the different layers there must be a 

 difference in composition to occasion it. The difiiculty of 

 obtaining glass of uniform composition, through a tendency to 

 separate into unlike portions, is one well known to makers of 

 lenses and other users of optical glass and has been frequently 

 described. It needs no further mention here beyond the com- 

 ment that this case adds another to those which have been 

 cited by others as a proof that the instability of molten silicate 

 solutions furnishes a presumptive proof of the possibility of 

 magmatic differentiation on a larger scale. 



If we accept the statement, which cannot unfortunately be 

 now verified, that the black obsidian is really a part of the 

 same glass flow, a more striking instance of differentiation is 

 shown in that the clear glass is rich in magnesia and poor in 

 iron oxide while in the obsidian the reverse is the case. This 

 difference explains very clearly why diopside crystallized out 

 in the one and wollastonite in the other. But such a move- 

 ment, provided we suppose the original molten solution to 

 have been homogeneous, whereby magnesia and ferrous oxide 

 are concentrated in opposite directions, while one would hesi- 

 tate to say it was impossible, from all our experience gained 

 in the study of rock-masses in which these oxides move together, 

 seems very unlikely to "say the least. And on the other hand, 

 we do not know, in the production of glass on so large a scale, 

 whether a sufficiently high temperature was maintained long 

 enough to permit the molten solution to assume conditions of 

 uniformity throughout its mass, if, on account of a difference 

 of materials used in the making, uniformity was not present to 

 begin with. Yiewed from this standpoint, it is possible that 

 the obsidian was part of the same flow. At all events, the 

 material seems to indicate the possibility of obtaining in this 

 direction, artificially, some light on the, at present, mysterious 

 process, or set of processes, known as differentiation. 



Oo')nposition of Minerals. — The use of the terms diopside 

 and wollastonite in this paper is of course only a general one 

 and it is not assumed that these substances are necessarily 

 present in a pure condition. From the splendid results which 

 have been achieved in recent years by Dr. Day and his 

 co-workers in the Geophysical Laboratory of the Carnegie 

 Institution concerning the properties of the lime and magnesia 

 silicates, and the conditions under which they are formed, and 

 Am. JouK. Sci. — ForRTH Series, Vol. XXX, No. 176. — August, 1910. 



