656 



ERUPTIVE BOCKS FROM MONTANA—MERRILL. 



lographic forms, but occur in comparatively broad, very irregularly 

 outlined plates surrounding and inclosing blebs of olivine and portions 

 of the grouudmass. The bronzite phenocrysts show a very platy, at 

 times almost fibrous, structure, and have as a rule bronze clouded 

 interior areas surrounded by colorless margins. 



This tyi^e of the rock passes quite abruptly into serpentine, the 

 olivines succumbing first and the bronzite next, the hornblendes 

 remaining intact to the last but finally going over to fibrous tremolite 

 forms. Accordingly as olivine or bronzite predominated in particular 

 instances, the sections show a serpentine of the well-known mesh, or 

 bar and grating structure. 



As the process of alteration is traced into the more schistose por. 

 tions of the rock it is observed that the unaltered hornblendes assume 

 an approximately parallel arrangement among themselves, their longer 

 axes lying in one general direction, while a pale yellowish mica is in 

 some cases developed, particularly in slides from specimens taken from 

 near the contact with the gneiss. The appearance is such as to sug- 

 gest that the apparent fissile structure is due to a lateral comi)ressive 

 or sheering force as in ordinary roofing slates, and that the force may 

 have been produced by movements in the inclosing gneiss, or, as seems 

 possible, to merely the expansion of the mass of rock itself during the 

 process of hydration and while held firmly by the walls of gneiss. 

 The sufidciency of this expansive force to produce a platy and slicken- 

 sided structure in pyroxenic masses undergoing hydration the present 

 writer has elsewhere alluded to.* The following analysis, by myself, 

 shows the composition oi the fresh, unaltered saxonite from the south- 

 ern end of the dike, all the iron being determined as FcaOa and the 

 rarer elements not looked for : 



Per cent. 



SiO, 



AUh 



Fe„03 



MgO' 



CaO 



Ign 



Specific gravity 



46.35 



16.41 



9.91 



18.72 



6.14 



3.01 



100. 54 

 3.21 



Pyroxenite. — Outcrop in gneiss. On divide between Meadow and 

 Granite creeks. Macroscopically this (No. 73175, U.S.N.M.) is a mass- 

 ive holocrystalline granular rock in which stout, deep dark-green, nearly 

 black, crystals of a hornblendic mineral in sizes up to five and eight 

 mm. in length are interspersed with larger indefinitely outlined areas, 

 .sometimes 40 or 50 mm. in diameter, of a brownish eminently cleavable 

 mineral, suggestive of a pyroxene. These two minerals, so far as can 



* On the Serpentine of Montville, New Jersey. 

 105. 



Proc. U. S. Nat. Mus., XI. 1888, 



