Weed and Pirsson — Bear paw Mountains, Montana. 361 



definition would apply as well to augite-syenite, or gabbro 

 could be called a peridotite rich in plagioclase. A pyrox- 

 enite, according to our conception, is essentially a non-f eld- 

 spathic rock, and we desire to emphasize at this point that 

 in the shonkinites described by us the alkali feldspars are a 

 constant and essential component, true pyroxenites, the practi- 

 cally non-feldspathic member of the series, never having been 

 observed at any of the localities. 



As a matter of fact, if the syenites be estimated as a series 

 practically free from plagioclase and we adopt Brogger's mon- 

 zonite group as an alkali-feldspar- plagioclase series as he defines 

 it, then the shonkinites are the syenite equivalents of the olivine 

 monzonites, while the pyroxenites — the end terms — are alike 

 in each, the two series approaching as the feldspars diminish 

 until they unite in the last term, the pyroxenites. 



This makes the fourth occurrence of this rock type described 

 by us from widely separated and distinct localities in central 

 Montana. We have material, also, from others which we 

 hope to describe at a future date. It is of interest to note that 

 theralite (a plagioclase-nephelite, granular rock) which shonkin- 

 ite strongly resembles in the character and abundance of the 

 ferro-magnesian minerals, also occurs in this region, in the 

 Crazy Mts., but appears to be a very rare type, the shonkinite 

 being a more common and dominating one.* 



Differentiation at the Beaver Creek core. — To enter into 

 detail concerning the facts of the differentiation of igneous 

 magmas at the Beaver Creek core and their bearing on theo- 

 retic petrology, would be merely a repetition of the discussion 

 given in our former papers. We have thought it best, there- 

 fore, to merely present the analyses (p. 362) in comparison with 

 those at Yogo Peak and let the figures tell their own story. 

 The lower figure in the case of each oxide refers to the Beaver 

 Creek, the upper one to the Yogo Peak series. 



At Yogo Peak the series began with a quartz-syenite por- 

 phyry^ and next to it came the syenite. The contact, if any, 

 between them is covered, and we cannot tell whether the 

 quartz-syenite porphyry is a differentiation in place, as seems 



* While this article was passing through the press we received a paper on 

 " Malignite, a family of basic plutonic orthoclase rocks rich in alkalies and lime," 

 by Prof. A. C. Lawson (Bull. Univ. California, vol. i, pp. 337-362, March, 1896). 

 These rocks appear closely related chemically and in part mineralogically to the 

 theralites and shonkinites, and although the author does not appear to recognize it 

 they clearly belong in Rosenbusch's theralitic magma series (Mass. Gesteine, 3d 

 ed., p. 385, 1895). 



f The analysis of the quartz-syenite porphyry given is not that of the rock 

 occurring at Yogo Peak but of a precisely similar type from Big Baldy Mountain, 

 a few miles distant. It represents perfectly, however, the Yogo Peak differenti- 

 ation. 



Am. Jour. Sci.— Fourth Series, "Vol. I, No. 5. —May, 1896. 

 24 



