180 Washington — Igneous Rocks from Eastern Siberia. 



In thin section the rock is seen to be noncrystalline, com- 

 posed almost entirely of irregular quartzes and orthoclase 

 grains, the latter showing a tendency to automorphic develop- 

 ment as stout prisms. A micropegmatitic intergrowth of the 

 two is quite common. The feldspar is uniformly clouded with 

 a brownish ferritic dust which gives rise to the megascopic 

 color. Scattered through the mass are irregular shreds of a 

 dark green aegirite-augite, with magnetite grains here and 

 there. Neither zircon nor apatite was seen. 



The crystallization of the NaFeSi 2 6 molecule as acmite 

 rather than riebeckite, in such siliceous rocks, is of compara- 

 tively rare occurrence, as noted by Pirsson in describing the 

 apparently analogous aegiritegranite-porphyry of the Judith 

 Mountains, Montana.* 



I. II. III. IV. v. VI. 



Si0 2 75-44 76-49 74'76 74-35 69-91 68'95 



A1 2 3 11-98 11-89 11-60 8*73 13-76 14*00 



Fe 2 3 0-88 1-16 3*50 5'84 2'17 2'12 



FeO 1-02 1-56 0-19 1*00 1-23 3*56 



MgO 0-10 trace 0*18 0'07 0*46 0*07 



CaO 0-33 0-14 007 0-45 1'39 0'23 



Na 2 4-06 4-03 435 451 4'45 5 '45 



K 2 5-01 5-00 4-92 3'96 • 6'33 5'29 



H 5 0+ .._ 0*68 0-38 0-64 0*25 0'12 0'05 



H 2 — ... 0-13 0-12 none 



C() 2 none none 



Ti0 2 trace trace trace 0-16 0-35 



P 2 5 trace 0*11 



MnO- trace trace trace 0*22 trace 0*55 



99- 



100-77 100-21 



99-38 100-09 100-62 



I. Comendite. Iskagan Bay, Siberia. H. S. Washington 

 anal. 



II. Paisanite. Magnolia, Mass. H. S. Washington anal. Jour. 

 Geol., vol. vii, p. 113, 1899. 

 III. Comendite. San Pietro, Sardinia. M. Dittrich anal. Rosen- 

 busch. Elemente, p. 237, 1898. 



IV. Grorudite. Varingskollen, Norway. Sarnstrom anal. Brog- 

 ger, Eruptivgesteine Christianiagebietes, i, p. 48, 1894. 



V. Aegirite-granite. Miask, Ural Mts. H. S. Washington anal. 

 VI. Arfvedsonite-grorudite. Fron, Norway. V. Schmelck anal. 

 Brogger, op. cit., i, p. 139, 1894. 



A chemical analysis of this Iskagan Bay rock is given in I 

 above, with a few of the very many analyses which might 

 have been chosen for comparison, in II, III and IV. It is 



*L. V. Pirsson, Eighteenth Ann. Rep. U. S. G. S., Pt. Ill, p. 559, 1898. 



