SPHERULITES AND LITHOPHYS.^ 271 



tion on the face immediately adjacent to it; thus in figure 6 we observe 

 in the lithophysa on the left a sharply pointed facet on the diagonal rib 

 in the lower left-hand corner of the vertical face; this facet appears on 

 the horizontal face in precisely the same relative position and is shown 

 faintly in the photograph. Much better examples of this phenomenon 

 can be seen in the lithophysfe on the right side of figure 6, but they are 

 not well reproduced in the photograph. The fact that any irregularity 

 on the one face finds its counterpart on the face adjacent to it and inter- 

 secting it at the edge of the cube proves that the faces were originally 

 togethei- and were grMdiially forced apart as crystallization proceeded. 



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FiGuui; S. — Enlarged central I'art uf Figure 7 

 Shows (loci'oase in sranularity away from center. Specimen S84.'^l. Magnification, 10 X 



Such pushing apart of a spherulite by the gas emitted during crystalliza- 

 tion or ])y tbe pulling apart as a result of general hydrostatic tension is 

 not unusual and would ordinarily be j)assed over unnoticed. 



In tbe present instance the remarkable synunetiy of the lithophysa' 

 attracts attentioii, ami the observer iinds it diilRcult to pictin'e the mech- 

 anism of such a jjrocess. When it is realized, however, that a cube can 

 be considered to consist oT a set of six four-sided pyramids (figure S>, 

 negative tetrahexahedrons mei'ting in the center at theif apices, the six 

 cube faces being theif hases), the geomeli'v of the |ii'ol)leiu becomes vlv.w. 

 \\\ then, having starle(| with a small s|ihei-nlite and liaviii^' caused ii in 

 fi-acture s\ ininei I'icall V as a I'esull of the iidei'iial gas [•I'essni-c idong the 



