WARREN AND PALACHE. — QUINCY PEGMATITES. 



167 



pairs ; thus the faces of (101) which are in zone with these unit pyramid 

 faces must be parallel to the twin plane. 



This twin law has been observed but once before on this mineral, on 

 crystals from the titaniferous calcite-quartz veins of Somerville, Mass.^^ 

 There twins were extremely rare, while here they are sufficiently 

 numerous to be considered characteristic for the locality. Combina- 

 tions of prism and unit pyramid are far the most common among these 

 crystals. A few, however, show the base as a tiny facet and in a few 



in 



m, 



Figure 10. 



Figure 11. 



the flatter pyramids k or z replace the acute summit of the common form. 

 Figures 10 and 11 illustrate the habit of the octahedrite crystals. 



Fluorite. — Mention has been made in preceding pages of the distri- 

 bution of fluorite throughout all parts of the pegmatite masses. It is 

 generally in small grains but near the central pocket, especially in that 

 part where crocidolite was abundant the fluorite individuals were larger, 

 one mass showing cleavage faces nine inches across having been found. 

 Where wholly embedded in crocidolite the fluorite crystals are automor- 

 phic, octahedrons up to one inch in diameter thus occurring ; they 

 are dull and somewhat rounded, the color a deep purple like all the 

 fluorite of this locality, but occasionally there is a surface layer of 



*' On Octahedrite, Brookite, and Titanite from Somerville, Mass., C. 

 Palache, Rosenbusch Festschrift, 1906, 311. 



