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DEPARTMENT OF THE INTERIOR 



2 GEORGE V„ A. 1912 



has showed, in fact, that the ealcite of many hundreds of amygdules together 

 composes a single crystallographic individual. The appearance of such an 

 interrupted individual is very similar to that of a coarsely poikilitic structure 

 in a plutonic rock. There is, of course, nothing more than an analogy between 

 the two cases, since the ealcite crystallized from infiltrating water, but the 

 parallel will serve, perhaps, to make the phenomenon better understood by the 



Figure 10. -Diagrammatic drawing to scale, from thin section of 

 amygdaloidal basalt in the Sheppard formation, Clarke Range. The 

 section shows twenty-four vesicles rilled with ealcite. The uniform 

 orientation of the ealcite is shown by the parallelism of cleavages (and 

 by simultaneous extinction under crossed nicols). The basaltic ma- 

 trix in which the amygdules lie (diabasic and very finegrained) left 

 blank. Diameter of circle 20 mm. 



reader. The poikilitic ealcite crystals are often of great size, diameters of 10 

 cm. (about 4 inches) being observed; in such cases the fillings of several 

 thousand vesicles compose a single crystal of glassy ealcite. The average 

 crystal is about 5 cm. in diameter. These large crystals never appear to 

 possess crystal outlines but lock together irregularly or are bounded by quartz 

 amygdules and non-vesicular parts of the rock. (Figure 10.) 



The nature of the process by which the carbonate was thus crystallized 

 offers an interesting problem, as yet unsolved. Most of the amygdules show 



