666 REVIEWS 



eruption. The fourth group includes basalt, which occurs only in 

 two localities, as extrusive lava. 



In the chapter on general petrology of the region it is pointed out 

 that the average composition of all the igneous rocks observed would 

 be that of a moderately acid syenite approaching an acid monzonite 

 in character. The rocks of the larger laccoliths present a somewhat 

 striking similarity in chemical composition and texture. They are 

 generally phanerocrystalline porphyries. They correspond to Rosen- 

 busch's granite porphyritic dike rocks. In this region they are pre- 

 eminently laccolithic rocks. Their more acidic character must be taken 

 into consideration in this connection, for in other regions less acid 

 rocks occurring in the form of laccoliths exhibit typical granular, non- 

 porphyritic texture. Here, as in other mountain groups of laccolithic 

 character in the Rocky Mountain region, the depths at which the 

 magmas are intruded appear to have exerted no perceptible influence 

 on their granularity. It is evident that chemical composition is an 

 important factor in the production of rock textures. 



Differentiation of igneous magmas and the formation of aplitic 

 veins are discussed and the variation in the mineral composition of 

 the rocks at Yogo Peak is expressed diagrammatically. The applica- 

 tion of diagrammatic methods to the discussion of chemical variations 

 among rocks of one district is considered with special reference to 

 Yogo Peak and the surrounding region. A comparatively simple 

 mathematical relationship is made out for the principal rocks of the 

 region, which is the more surprising when the intricate nature of the 

 chemical molecules of several of the rock-making minerals is con- 

 sidered. It is perfectly evident, as an abstract proposition, that the 

 chemical composition of any rock is a mathematical function of the 

 several component minerals, whose chemical molecules are more or 

 less variable functions of a few chemical elements. From which it 

 may be inferred that whatever the process by which differentiation of a 

 magma takes place the resulting solutions or magmas will probably 

 sustain a mathematically intricate functional relation to one another. 

 In the present instance the approximate relations appear to be com- 

 paratively simple. It is to be noted, however, that the correspondences 

 between observed and estimated composition presented by Professor 

 Pirsson, as he himself remarks, are merely close approximations. They 

 are, nevertheless, striking. With regard to the absorption of sedi- 

 ments by magmas, the study of the igneous rocks in the Little Belt 



