JPirsson — Petrograpliy of Tripyramid Mountain. 417 



The olivine is clear and colorless, unaltered save for a slight 

 serpentinization along cracks ; while at times it is shapeless, 

 and fills spaces between other constituents which determine its 

 form, in other cases it shows more distinct crystal outlines ; it 

 is often included in feldspar in minute grains ; the only inclu- 

 sions are occasional granules of iron ore and specks of hiotite, 

 but in some places shadowy spots under high powers reveal 

 themselves to be thin sheets of magnetite, which when seen on 

 end appear as lines, but viewed flatwise are found to be skele- 

 ton crystals presenting remarkable patterns of grating struc- 

 tures, such as have been described by petrographers in olivines 

 from various localities.* 



The olivine of this rock was analyzed by E. S. Danaf on 

 extracted grains with the following results : 



A. B. C. D. 



SiO, 38-85 0-647 0'050 0-597 0-597 =1'04 



FeO 28-07 0-389 0-012 0-377 



MnO 1-24 0-017 0-017 J- 0-1146 = 2-00 



MgO 30-62 0-765 0-013 0"752 



CaO 1-43 0-025 0-025 



A1„0„ trace 



Total ... 100-21 



Since the amount of alumina is practically nothing the lime 

 must be due to a little admixed pyroxene. If, from the molec- 

 ular ratios shown in B, a sufficient amount of iron, magnesia 

 and silica be deducted, as in C, to convert the lime into pyrox- 

 ene, the final ratios available for olivine are given in D, which 

 gives an excellent result for the olivine formula. There is an 

 unusually large amount of iron present, the mineral being 

 practically Fe a Si0 4 .3Mg 2 Si0 4 , and thus belonging to the variety 

 hyalosiderite, which explains the presence of iron ore sepa- 

 rated out along the cracks, where alteration has taken place, 

 and thus confirms Rosenbusch's:}: view that in such cases the 

 olivine of gabbro rocks is one with a large content of iron. 



The iron ore is shapeless, and only in one section was dis- 

 tinct crystal outline seen, that of a hexagon. It is observed in 

 relatively large masses and in small grains ; the large masses, 

 like those of the pyroxene, have their form conditioned by the 

 surrounding minerals, and in a number of cases the ore was 

 found completely enclosing automorphic labradorite. This 

 shows that the period of crystallization for the ore lasted long 

 after that of the basic labradorite, a phenomenon which Eosen- 



* Petrography of the Little Belt Mts., 20th Ann. Rep. U. S. Geol. Surv., 

 (Pt. Ill, p. 481, 1900). 

 f Loc. cit. 

 I Massigen Gesteine, 4 te Auft. 334, 1907. 



