PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 191 



I ,;U>r:ulurite.] 



CJiemical Analysis. The following analysis was made by Prof. J. A. Dodge, and first published in the 

 Thirteenth Annual Report, page 100 (No. 156): 



SiO 2 - - 71.99 



A1 2 6 3 12.36 



Fe 2 O 3 4.99 



FeO .56 



CaO .85 



MgO .72 



K 2 O 2.45 



Na 2 O .99 



H,O - - 2.92 



Total - 97.83 



Age. Cabotian. 



HnitiirL-x. The groundmass of the rock is identical with that of some altered 

 obsidians, and the bands may be well referred to original chains of spherulites. The 

 rock is here referred, without much hesitation, to a devitrified obsidian; /. c., an 

 apobsidian. This rock is undoubtedly the same as No. 528. u. s. G. 



No. 128. LABKADORITE. ( Crysldh. ) 



Near the mouth of Beaver creek, a few rods to the north of it; large feldspar crystals formerly weathered 

 out of a crumbling block of the gabbro, lying at the upper side of the beach. These crystals were within fifteen 

 feet of another upheaved outcrop of rock like No. 127. 



Ref. Annual Report, ix, page 33; Annual Report, x, page 139; Bulletin Societe" Francaise de Mine'ralogie, 

 vol. xix, page 90; American Association for the Advancement of Science, vol. xxx, page 162. 



The crystals are large, more or less fragmentary, only one being sufficiently 

 entire to show some of the faces of the form. This is represented by the following 

 figures. A larger crystal was broken and cut for chemical and for microscopical 

 examination. The crystal examined was at first thought to be a simple crystal, but 

 on measuring the angles with a common goniometer it was found not to agree with 

 that supposition. Further search revealed the presence of two directions of easy 

 cleavage, thus indicating the two bases (001) of a Carlsbad twin. The other, larger, 

 crystal showed the line of junction of the twins much more distinctly. From this a 

 powder was produced, the thin cleavage pieces showing very perfectly not only the 

 purity of the crystal but the following extinctions: 



Extinction on 010, 25 to 27 (about 26). 



Extinction on 001, 7 to 11 (about 9). 



Specific gravity, 2.72 (in methyl iodide). 



Test by the Boricky method gave numerous microliths of fluo-silicate of calcium, 

 and a few of sodium. 



Some of the fine powder was boiled for over an hour in hydrochloric acid. The 

 fragments were somewhat affected, but the larger grains still polarized brilliantly 

 between crossed nicols. Even the smallest retained their forms. In the platinum 

 crucible this powder was mingled with an aniline color (vert de methyle) in solution 

 in water, in order to show, by the permanent coloration, the presence of gelatinous 



