670 



ERUPTIVE ROCKS FROM MONTANA— MERRILL. 



greenish cbloritic substance, and in part a colorless substance so tlior- 

 ouglily impregnated witli minute specks of opacite as to give it a blnish 

 hue, remotely resembling bauynite. This secondary substance acts 

 faintly on polarized ligbt, and, being insoluble in boiling acids, is pre- 

 sumably chalcedony. Were it not that tlie outlines of these pseudo- 

 morphs are idainly those of olivuies, and the amount of this insoluble 

 substance increases proportionally with the alteration the crystals have 

 undergone, I should hesitate to designate them as olivine derivatives. 



Although this rock shows certain structural i)eculiarities, differing 

 from that just described, I am at present disposed to regard it as a 

 portion of the same flow solidifying, it may be, under slightly different 

 conditions, and having undergone greater changes since its eruption. 



Below are given the results of analyses on the three types indicated, 

 Nos. I and iii being by Mr. L. G. Eakins, and Xo. ii by Dr. Ghatard, 

 of the U. S. (leological Survey. No. i is the fresh porphyritic variety 

 from South Boulder and Antelope Greek (No. (JUiOO, IJ.S.N.M.); JSTo. il 

 the variety collected in 1886 from near Gottonwood Greek (No. 38596, 

 U.S.N. 31.); No. Ill the variety found in nodular masses in decomposed 

 material as just described, and No. IV a rock from the Absaroka range, 

 as described bv Iddings.* 



SiOj .. 

 TiO.,. 



SO3.-. 



Cr.,Oj 

 Fe;0, 

 FeO .. 

 MnO . . 

 CaU... 

 MgO.. 

 K,0.. 

 Na.^O . 



r.ao .. 

 H20 .. 



Iffn . . 



Specific gravity. 



I. 



Per cent. 



■ 50.82 

 .59 

 .20 



11.44 

 .03 



.25 



8.94 



.19 



8. U 



14.01 



:i. 45 



1. 70 

 .06 



.58 



100. 49 

 2.96 



II. 



III. 



IV. 



J'er cent. 



a51 . 65 



.55 



.21 



.19 



13.-89 



.80 



2.70 



4.80 



.15 



4.07 



11.56 



4. 15 



2.99 



. 19 



1.30 



1. 80 



Per cent. 



50.03 



.61 



.42 



Per Cfvt. 



48. 36 



1. 18 



0.84 



14.08 



Trace. 



2.92 



6.11 



.08 



7.46 



10.73 



2.64 



1.46 



.04 



3.70 



12.42 

 Trace. 

 5. 25 

 2. 48 

 0. 13 

 8. 65 

 9.36 

 3.97 

 1.46 

 0.29 



I 5.54 



a An analysis of the portion of No. II soluble in liydrochloric acid 

 subsequently yielded results as follows: 



Tbe soluble portion yielded : 



SiO-i 



A1,0, 



ye.jOi and FeO. 



MnO 



CaO 



MgO 



K.,0 



NajO 



Per cent. 



41.87 

 9.48 



16.41 



Trace. 



3. 10 



26. 79 

 1.06 

 1.26 



09. 97 



■ The Origin of Igneous Rocks, Bull. Philos. Soc. of Washiugtou, Vol. xii, 1892, p. 169. 



