eG6 



ERUPTIVE ROCKS FROM MONTANA— MERRILL. 



The determinations were not duplicated, and can be regarded little 

 more than suggestive. 



1 Per cent. 



SiO-i 60. 68 



ALO3 aud Ve.jOi iil- 71 



CaO I 3.03 



K2O i 7.31 



Na,0 (bv iliftereDce) i 6.63 



99.36 



Evidently a mixture of potash and soda-lime feldspars. A bulk 

 analysis of the rock as made by L. G. Eakins, of the U. S. Geological 

 Survey, yielded the results given in column i, on p. 070. 



In the dry ravines and gulches near by, and on the north side of the 

 road leading from Antelope to South Boulder Creek were found obscure 

 outcrops of what from its position was assumed to be the same rock, 

 but which in a state sufficiently fresh for examination could be found 

 only in small rounded bowlders, the main mass of the rock having so 

 thoroughly rotted as to be easily dug out with the hand pick. The 

 freshest nodules obtained showed on a broken surface a deej) dark 

 greenish gray indistinctly porphyritic rock in which olivines and aug. 

 ites, with occasional tiecks of brown mica, are determinable by a pocket 

 lens. In the tliin section this variety is nearly if not quite holocrys- 

 talline but its structure badly obscured by decomposition. A clear 

 glassy sanidin intergrown with plagioclase is readily made out, green 

 augites, brown mica, and badly altered olivines. 



During a previous season (188()) small outcrops of a somewhat simi- 

 lar rock were found near Cottonwood Creek and east of the Gallatin 

 River in Gallatin County. These on comparison proved to be undoubt- 

 edly portions of the same magma, but offer some interesting peculiari- 

 ties. 1 find these desciibed as follows in my notes of the winter of 

 1880-'87 : 



Macroscopically this rock (No. 38590, U.S.N.M.) consists of a compact 

 aphanitic, dark gray or nearly blacli, sometimes brownish, groundmass 

 in wliich are embedded abundant dark green porphyritic olivines and 

 augites of all sizes up to five millimeters in greatest diameter. 



Microscopically the rock is both unique and beautiful. In a dense 

 groundmass of a light gray, sometimes brownish color, consisting of a 

 colorless or graj' undeterminable mineral, augite microlites, small scales 

 of brown mica, and grains of iron ore, are embedded beautiful large clear 

 grains of olivine and augite, these two minerals alone constituting the 

 porphyritic ingredients. 



The olivines occur in clear, colorless, rounded, and irregularly cor- 

 roded forms, scattered singly or in polysomatic groups, as shown in 

 figure 8, and often in close juxtaposition with the augites. They are 

 beautifully clear aud fresh, with but few inclosures of magnetite and 



