1894. 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



641 



It is evident that tins is essentially olivine with a mixture of iron 

 oxides and decomposition products. 



In the summer of 1889, while working on Bear Creek, in the foothills 

 just east of and overlooking the Madison Valley, some 45 miles in a 

 direct line to the southwest, inconspicuous outcrops of an intrusive were 

 discovered, which were at once seen to be nearly identical. On return- 

 ing to Washington thin sections were prepared, from an examination 

 of which the first impressions were abundantly confirmed. Certain of 

 the slides were indistinguishable from those of the Fort Ellis rock; 

 others differ in showing a groundmass more crystalline and a some- 

 what smaller proportion of tlie porphyritic olivines. The base prox)er 

 is here a nearly colorless glass occurring only in the interstices of a 

 crowd of small, lath-shaped feldspars, mainly a i^Iagioclase variety, 

 though certain nonstriated forms may be sauidin. 



Throughout this groundmass are scattered innumerable black gran- 

 ules of iron ore and the olivines and augites as already described. The 

 rock has an aspect more nearly like that of normal basalt, but difi'ers 

 in the character of its pyroxenic constituent and the abundance of its 

 olivines. A bulk analysis of the rock by Dr. Chatard yielded results 

 as in I below, n is the Fort Ellis rock reproduced for the purposes of 

 comparison. 



The close relationship of this rock to the dark eruptive between 

 South Boulder and Antelope Creek is mentioned on page 673. 



Augite andesite. — Hills east of Fort Ellis. As here displayed, this is 

 a coarse, dark-gray rock (No. 38597, IT.S.lSr.M.), full of vesicles and 

 amygdules of all sizes up to an inch or more in diameter. Its only 

 macroscopic constituent, aside from the secondary minerals comprising 

 the amygdules, is a dark-greenish augite which occurs as scattering 

 crystals, at times four or five millimeters in diameter. 



Under the microscope the rock presents an exceedingly dense micro- 

 litic groundmass of lath-shaped plagioclases, augites and iron oxides 

 in which are embedded widely-scattered porphyritic plagioclases, 

 and more numerous augites with an occasional dusky apatite. The 

 augites, although comparatively fresh appearing, are rarely in well- 

 developed crystals, but occur as very irregularly corroded and rounded 

 forms full of inclosures of the base and of magnetite particles. In the 

 Proc. N. M. 94 41 



