270 



F. E. WRIGHT OBSIDIAN FROM ICELAND 



is as clear in the Icelandic material as in the Yellowstone Park occur- 



rences. 



Specimen 88431. — In this specimen we encounter lithophysa; of a 

 shape and aspect which are unique. They are so remarkable that at first 

 sight they do not appear to be spherulites. The cavities are in the shape 

 of a cube about 25 mm. on a side, the walls of the cube having the ap- 

 pearance indicated in figure 6 and in the photomicrographs, figures 7 

 and 8. The inner walls are not perfectly flat, but show strong diagonal 

 ribs passing from one corner of a cube face to the opposite corner, as 

 though each cube face had not been quite fully developed into a i)lnii('. 



y .. 



FniuUE 7. — Lower Wall of LiUioijIiyna on lefl aide of lUjure 

 Shows character of crystallization. Specimen 88431. Magnification, 4 X 



but still has superimposed on it four negative triangular tetrahexahedral 

 faces; each four-sided pyramid thus formed points toward the center of 

 tlie cube and not away from the center, as in the case of natural crystal 

 l»ounded by tetrahexahedral faces. Between the strong diagonal ribs 

 iiollow depressions occur. From the apex of each four-sided pyramid 

 fibers radiate toward the sides of the cube, as shown in figure 6. In addi- 

 tion to these lines of growth, a second set of structural lines and ridges 

 and cracks is present, cutting the radiating lines at right angles and 

 emanating as encircling waves from the center (see figure 7). 



On examining these cube faces still further, we find that any little 

 in-egularity on the one face is imaged in exactly the same relative posi- 



