356 Merwin — Simultaneous Crystallization of Calcite, etc. 



The second specimen is a calcareous geocle about 9 inches in 

 diameter, from central Missouri. The geode was tirst lined 

 with dark brown calcite containing bituminous (?) matter. 

 With this is associated a very little marcasite in free spaces 

 amongst the calcite. A second generation of clear calcite is 

 accompanied by considerable marcasite and some pyrite. The 

 marcasite appears in slender blades, or reticulate structure due 

 to twinning. It is partly free and partly enclosed in calcite. 

 Much of it appears to have formed since the clear calcite 

 began growing, but conclusive evidence is lacking. 



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mmhvi 







Fig. 1. 



The pyrite crystals, all similarly oriented upon the marcasite 

 blades, appear like knots or encircling beads. The marcasite 

 has not grown perceptibly since the pyrite began depositing, 

 but it is still bright. Some pyrite crystals have been enclosed 

 in calcite. Others after being partly enclosed grew out over 

 the calcite, indicating a halt in the growth of the calcite 

 (fig.l.D). 



There is no evidence that during any interruption in its 

 growth the calcite was etched. After etching, there would be 

 a strong tendency for any renewed growth to start irregularly 

 upon the old, giving rise to a parallel aggregate of small 

 crystals rather than to the single symmetrical crystals found on 

 the specimen. 



A thin section shown me by Mr. J. B. Umpleby contains a 

 minutely-branching crystal or a parallel aggregate of wurtzite 



