The American XntumHtt. 



[April, 



tendency toward the allotriomorphic structure. Further west, about 

 in the center of the mass, the syenite changes to a darker gray rock 

 with a tinge of green, somewhat resembling a diorite. It is more 

 coarsely crystalline than is the syenite and is much more basic. The 

 minerals are the same as in the syenite, except that quartz is lacking, 

 but differ somewhat in their character and in the proportions present 

 in the two rocks. The augite is now a bright green idiomorphic min- 

 eral. Hornblende is rare and biotite abundant. The great difference 

 between this rock, which the authors call yogoite, and the syenite, is in 

 the relative proportions of augite and orthoclase present in them. In 

 the yogoite the pyroxene predominates over the orthoclase, while in 

 the syenite the reverse ratio exists. In the western portion of the rock 

 mass, the prevailing type is shonkinite, a very dark basic rock, very 

 similar to that of Square Butte. 3 Augite and biotite are very abun- 

 dant as compared with the orthoclase, which in turn predominates over 

 plagioclase. This latter mineral is represented by andesine, a more 

 basic feldspar than that in either the syenite or the yogoite. Analyses 

 of the three types of Yogo Peak rocks follow : 



SiG 2 Ti0 2 AI 2 3 Cr 2 3 Fe 2 3 FeO MnO MgO CaO BaO SrO Na 2 K 2 H 2 P 2 6 



From a consideration of the nature of the three types of rock the 

 authors conclude that the Yogo Peak stock exhibits the results of a 

 progressive differentiation along its major axis. There is a progressive 

 increase in the ferro-magnesian constituents from the east to the west 

 and a consequent increase in basicity. All the components of the three 

 types exhibit the effects of this differentiation in the proportions pres- 

 ent in the different rocks. The Yogo Peak mass is thus an illustration 

 of a " Facies suit " as distinguished from a " rock series." In the for- 

 mer differentiation took place in situ, whereas in a ' rock series ' differ- 

 entiation occurred before the eruption of rocks into their existing posi- 

 tions. The facies suit of Yogo Peak together with the rocks of 

 neighboring mountains comprise a distinct rock series. 



The authors close their paper with an appeal for a more specific 

 nomenclature in petrography— a nomenclature that will take account 

 not only of the qualitative relations between the minerals that make up 

 rock masses but of the quantitative relations as well. The Yogo Peak 



3 Compare American Naturalist, 1895, p. 737. 



