556 Pal ache and Warren — Chemical Composition and 



zircon; and they occur in cruciform twin groups with the form 

 (101) as twin plane. The twins are sometimes complete inter- 

 penetrations of two equal crystals as shown in the figure ; some- 

 times but one end .of each is developed ; again a larger crystal 

 has a much smaller one in twin relation to it. The groups are 

 exquisitely sharp and leave no doubt as to the definiteness of 

 the twinning since the two upper faces of the unit pyramid of 

 each crystal and the two lower, parallel and opposite faces to 

 these, reflect the signal simultaneously in 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 as charac- 

 teristic for the locality. Combinations of prism and unit pyra- 

 mid are far the most common among these crystals. A few, 

 however , v show the base as a tiny facet, and in a few the flatter 

 pyramids Tc or z replace the acute summit of the common form. 

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



Fluorite. — Fluorite is distributed 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 show- 

 ing cleavage faces nine inches across having been found. 

 Where wholly embedded in crocidolite the fluorite crystals are 

 idiomorphic, 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 occasion- 

 ally there is a surface layer of bluish green color due to in- 

 cluded fibers of the blue crocidolite. The hollow castes left 

 by the solution of such crystals have already been described. 



In one or two cavities in the Fallon pegmatite there were 

 seen tiny cubes of fluorite implanted on quartz, and in another 

 such pocket, peculiar in containing also crystals of calcite, the 

 cube was modified by two hexoctahedrons which appear to be 

 new to fluorite. The measurements and derived symbols of 

 these forms follow : 



Calculated. 



Measured 



Limits 



> 



No. of 

 read'gs 



<t> P 

 f 3.10.16 16°42' 33°07' 

 -{ 3.16.10 10 38 58 26 

 1 10.16.3 32 00 80 58 



P 

 15°45' 33°20' 

 10 36 58 33 

 32 04 81 05 



4> 



10°30'- 10°42' 

 31 52 - 32 16 



58°30' 



80 50 



P 



-58 c 36' 2 

 -81 20 4 



f 259 21 48 30 54 

 -j 295 12 32 61 32 

 [592 29 03 79 00 



21 01 31 15 

 12 32 61 25 



20 45 - 21 18 

 12 27 - 12 37 



31 11 

 61 23 



-31 19 2 

 -61 26 2 



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

 Palache. Eosenbusch Festschrift, 190ft, 311. 



