Merwin — Simultaneous Crystallization of Calcite, etc. 359 



fact, the observed monotropic transformation of marcasite to 

 pyrite is accompanied by only a small evolution of heat and a 

 slight increase in density.* Yet experiments in this connec- 

 tion by Dr. E. T. Allen show that crumbling of the crystals 

 accompanies the change. 



The microscope shows that the faces of the dodecahedron 

 represent the edges of minute steps made up of octahedral 

 planes. Oblique planes of the cube and pyritohedron interrupt 

 the steps somewhat. The steps are sufficiently close together 

 to produce the semi-diffuse reflection noticed in the light 

 figures (fig. 2, A). Still closer spacing would lead to increased 

 brilliance of the light figures in the vicinity of the dodeca- 

 hedron, and very close spacing — closer than the length of waves 

 of light — would produce brilliant faces. 



The octahedral crystals have faces more or less convex 

 owing to the same step-structure present on dodecahedral faces. 

 If the steps on the octahedral faces were fine enough to make 

 the faces mirrors the slopes of the faces on different crystals 

 would be different and apparently would not lead to simple 

 symbols. 



The chain of signals usually seen in the goniometer telescope 

 between the cube and pyritohedron (210) comes from a series 

 of small faces made up of closely spaced steps. These faces 

 correspond not only to well recognized crystal forms but to 

 forms which have very complex symbols. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 Washington, D. C, July 13, 1914. 



* See this Journal, xxxiii, 188, 1912. 



