414 Pirsson — Petrography of Tripyramid Mountain. 



Chemical Composition. — The rock was analyzed by Mr. C. 



J. Monahan with the result given in No. I. 



I. II. III. IV. v. 



SiO, 48-67 49-46 50*47 48-06 0-811 



Al„O s 16-88 19-82 18-98 16-95 0-166 



Fe^O, 4-98 5-69 4-22 4-78 0*031 



FeO 6-37 5-82 6-16 7*60 0-089 



Mo;0 4-62 1-93 5-62 5-51 0*115 



CaO 8-63 10-62 11-72 7*79 0-154 



Na 2 3-85 3-38 2-75 3*37 0-062 



K0 2 1-26 0-74 0-56 1-42 0-018 



H 2 0+ 0-32 0-09 1-06 0-80 



H;0-.... 0-02 .... 



Ti0 2 2-12 0-69 0-12 2*57 0-026 



P„(X 1-85 ? ? 0-63 0013 



Total... 99-57 99-84 101*78 99-48 



I. Norite, andose, Slide Brook, Waterville, N. H. C. J. 

 Monahan, analyst. 

 II. Norite, hessose, Near Ivrea, Piedmont. (Van Horn, Tscher. 

 Mitt., xvii, p. 404, 1897.) M. Dittrich, analyst. 



III. Norite, Eperon du Cerebriansky, west side, Northern 



Urals. Duparc et Pearce, Mem. Soc. Phys. e. d'Hist. 

 nat. de Geneve, p. 466, 1905. 



IV. Bronzite-kersantite (andose), Hovland, Laugendal, Nor- 



way. (Brogger, Erupt. Gest. Krist. geb. Ill, Ganggef 

 der Laur. p. 75, 1899.) V. Schmelck, anal. 

 V. Molecular proportions of No. I. 



In studying the analysis it is clear that since a very con- 

 siderable proportion of the lime has been used to convert the 

 excess of alumina over soda into plagioclase, there is not 

 enough left to turn all the magnesia and the excess of the 

 ferrous iron, not used in iron ore, into pyroxene. These extra 

 magnesia and ferrous iron molecules have in this case formed 

 hypersthene rather than olivine, because there was an excess 

 of silica present, as the residential quartz shows. Thus a norite 

 is formed, rather than an olivine gabbro. In this connection 

 the reader may compare this analysis with that of the olivine 

 gabbro on a succeeding page, where, under No. VI, the norite 

 is repeated for convenience. The gabbro differs chiefly in 

 containing 3 per cent more of alumina and lime, and about that 

 amount less of ferric iron. Since 3 per cent CaO = 0-59, and 

 of A1 2 8 — 0*29, molecularly, a larger proportion of lime is 

 able to combine in pyroxene, a metasilicate, and, since the 

 lower ferric iron demands less ferrous to form iron ore, the 

 result is that, with the lower silica content, olivine is found 

 and not hypersthene, as in the present rock. 



