F. L. Ransome — Lava Flows of California. 



367 



XX. Hornblende-pyroxene andesite, Downieville Quadrangle, Cal. Tur- 

 ner, 17th Ann. Rept. U. S. Geol. Survey, p. 731. Hillebrand, 

 analyst. 

 XXI. Trachyte (Domite), Puy de Dome, France. Zirkel, Lehrbuch der Pe- 

 trographie, ii, p. 378. Lewinstein, analyst. 

 XXII. Quartz-banakite, dyke near head of Stinkingwater River, Yellowstone 

 Park region. Iddings, Jour. Geol., vol. iii, p. 947. Melville, 

 analyst. 

 XXIII. Quartz-banakite, dyke, locality and reference as above. 

 XXIY. Monzonite. Predazzo. Brogger, Die Eruptionsfolge der triadischen 



Eruptiogesteine bei Predazzo in Sudtyrol, Kristiania, 1895, p. 25. 

 XXY. Monzouite, Sasinathal, Predazzo. Brogger, loc. cit. 

 XXYI. Monzonite, Beaver Creek, Mont. Weed and Pirsson, this Journal, IY, 



vol. i, p. 357. Stokes, analyst. 

 XXYII. Monzonite, Monzoni, South Tyrol. Brogger, op. cit , p. 24. Schmelck, 



analyst. 

 XXVIII. Monzonite, Yogo Peak, Mont. Weed and Pirsson, loc. cit. Hille- 

 brand, analyst. 

 XXIX. Monzonite, Mulatto, Predazzo. Brogger, op. cit., p. 25. 



XXX. Monzonite, Malgola, Predazzo. Brogger, loc. cit. 

 XXXI. Monzonite, Predazzo. Brogger, loc. cit. 

 XXXII. Monzonite, Blansko, Malvren. Brogger, op. cit., p. 50. 

 XXXIII. Monzonite, Hodritsch, Hungary. Brogger, loc. cit. 



On the other hand, it is no part of pure petrographical sci- 

 ence to aim directly at ease and simplicity in classification, how- 

 ever desirable that end may appear to the general geologist. 

 Petrography in its present stage is occupied rightly with crit- 

 ical distinctions, and it is only by the recognition of such 

 slight points of difference that they can ever be transcended, 

 and assigned their true value in a really rational scheme of 

 rock classification. 



Recognizing the fact that igneous rocks are chemically mix- 

 tures, of practically infinite variability within certain limits, 

 the refining tendency of modern methods tends more and 

 more toward precision of definition, and a consequent subdi- 

 vision of the older groups. This gain in precision by no 

 means makes the classification of a given rock an easier matter, 

 for it is evident that the greater the number of families, the 

 more numerous will be the intermediate or transition rocks, 

 since sharp lines of demarcation apparently nowhere prevail 

 between closely related types. It is a question of course how 

 far, for purposes of practical description, such subdivision of 

 large groups should go, but it is a question that may be very 

 well answered by the facts of geological occurrence. If a 

 magma of a certain intermediate type with reference to two 

 or more well established families is found to possess geological 

 unity, and particularly if it is found to be represented by both 

 plutonic and effusive forms, there can be little doubt of the 

 advisability of giving it a distinctive name. Brogger's mon- 

 zonite magma is an example of such a case, and he has shown 

 very conclusively that the plutonic monzonites, being ortho- 

 clase-plagioclase rocks, should be separated from the orthoclase 



