and its Spherulitic Crystallization. 105 



stituents have had no opportunity to be2:in to crystallize before 

 the mass stiffened. But experience also shows that, in gen- 

 eral, the more rapid the cooling the greater the number of 

 centers of crystallization which will be set up. Now if any- 

 thing should start to crystallize in magma under such cwidi- 

 tions it would be the magnetite, ordinarily the earliest mineral 

 of importance to form, and it would be therefore distributed 

 through the glass in the shape of the finest dust acting mega- 

 scopically as a mechanical pigment and coloring it black. The 

 red color that many obsidians show may then be due to the 

 complete oxidation of the magnetite to ferric oxide. 



On the other hand, a study of a number of sections of vari- 

 ous obsidians, and also of the literature, seems to indicate that 

 the color may not be wholly due to magnetite. Zirkel,* in dis- 

 cussing it, considers that it is sometimes inherent in the glass 

 (chemical so to speak) and is sometimes due to inclusions of 

 minute size, which he describes. In the latter case the glass 

 is colorless, and in the study of several obsidians of this char- 

 acter the writer has observed that the microlites, or trichites, 

 have a higher index of refraction than the surrounding glass, 

 as shown by Becke's method. On lowering the objective 

 beyond the focal point they appear black, on raising it above 

 they become illuminated and appear colorless. Consequently 

 they are transparent and not magnetite. The greater the 

 number of these incipient crystallizations there are, the blacker 

 and less transparent the glass appears megascopically. The 

 black color in this case then is due to light absorption. In 

 one case where the slender microlites were arranged in parallel 

 positions in streams, it was noticed that the section possessed a 

 distinct pleoehroism ; when the ray vibrated across these 

 streams it appeared colorless; when the long diameters were 

 parallel to the ray it was distinctly brown. This is an effect 

 of light absorption similar to that previously described under 

 spherulites and doubtless produced in the same way. The 

 black color of many obsidians then appears to be caused by 

 the dispersion, total reflection, and absorption of light due to 

 the presence of innumerable hosts of minute crystalline bodies 

 of a higher refractive index than the glass in which they lie, 

 such crystalline bodies being themselves colorless. 



In the case of the artificial obsidian of Kane, with magnifi- 

 cations of 5ttO diameters, the mici-oscope reveals the presence 

 of bodies of indeterminate nature. They are so minute 

 that no definite shape can be assigned to them, but they 

 give a vague impression of being rudely octahedral. On lifting 

 the objective they become strongly illuminated, on lowering 

 they appear black. Thus they are transparent and of a higher 

 *Lehrb. d. Petrog., vol. ii, p. 371, 1894. 



