102 L. V. Pirssou' — Artificial Lava-Flow 



where the fibers are finest and most closely packed ; at the 

 periphery, where they are coarser and more individualized, it is 

 wanting. With a very high magnification kA the center and 

 paralk'l light from below, it disapjiears. With low-powered 

 objectives it is present, whether the lower nicol is present or 

 not. The sphernlites of feldspars in acid volcanic lavas often 

 show the same }"»henomenon of brown coloration, and so do the 

 closely packed aggregates of minute scales of kaolin in feldspar. 

 Since in all these cases the component mineral fibers, or scales, 

 are white, that is to say colorless, this effect cannot be due to 

 a pigment, but must be an optical phenomenon due to the 

 absorption of light. An explanation of this may be somewhat 

 as follows : Angular fibers or scales of a colorless mineral 

 lying in all positions in a medium of lower refractive index 

 may act as prisms to rays of light passing through them. The 

 rays of white light would be then decomposed into their pri- 

 mary colors, and the violet rays more inclined than those of the 

 red. On passing upward and striking inclined surfaces of new 

 fibers, or scales, the light in the violet end of the spectrum, on 

 account of its greater inclination than that towards the red 

 end, would tend in greater measure to suflier total reflection 

 and be absorbed, while the less inclined rays of the red and 

 yellow would pass through and, illuminating the dark areas of 

 total reflection, give the fibrous mass the brown tone. This 

 explanation is based essentially on the same principle as that 

 given by von Federov* for the pseudo-pleoehroism which 

 he observed in minerals containing thin parallel laminations. 

 Only in this case there is no parallelism, the angular fibers lie 

 in all directions, hence there is no apparent pleochroism and 

 the mass appears of the same brown color in all positions and 

 with, or without, polarized light. 



It may be that in this lies the explanation of the brown and 

 gray colors and pleochroic eft'ects observed in spherulites in 

 certain volcanic glasses by Cole,f but a search through the lit- 

 eratui-e has not yielded to me any discussion of the frequent 

 brown color of spherulites (or kaolin) though so commonly 

 observed. Probably it is usually ascribed, if noticed, to some 

 ferruginous pigment, to which in some cases it is undoubt- 

 edly due. 



Some other features of interest which these spherulites show 

 are as follows: In some, especially larger ones, hollow cavities 

 appear; others have no cavities but a pronounced crack run- 

 ning through the center ; often this crack passes along through 

 several of them, and it is always rigorously parallel to the 

 direction which the chain or line of spherulites makes (line of 



*Mm. Petr. Mitt., xiv, p. 569, 1895. 



f Quar. Jour. Geol. Soc, xliv, p. 302, 1888. 



