188 University of California. [vol. a. 



plagioclase. By all the tests this plagioelase is found to be an 

 acid labradorite or basic andesine. I, while chemically more acid 

 than II, is slightly more basic mineralogically, as it contains a 

 small amount of basic labradorite. The conflict is apparent. 



Rock I may be classified without much hesitation as a basic 

 diorite, poor in feldspar, rich in hornblende, and with a very 

 little quartz. This corresponds to the nomenclature of Rosen- 

 busch, as cited in analysis IV. Rock II cannot be so easily 

 disposed of, however, although so similar to I. Such names as 

 hornblende- gabbro, gabbro-diorite, or diorite-gabbro naturally 

 suggest themselves. But in the two cases cited, the term 

 hornblende-gabbro is used to denote rocks containing hornblende 

 as a primary constituent, with augite or diallage and a basic 

 labradorite. Irving* uses the same term to denote a rock with 

 basaltic hornblende, augite, diallage, labradorite essentially, and 

 some accessory biotite, oligoclase, and quartz. This use of the 

 term seems well taken; it should be limited to such rocks. The 

 term gabbro-diorite, as used by Williams, analysis X, denotes a 

 gabbro in which the original pyroxene has been changed to 

 amphibole. In this case the term diorite-gabbro would appear 

 to be better. Brogger has suggested the terms diorite-gabbro 

 and gabbro-diorite for these rocks of intermediate type, but 

 inclines more to the purely chemical differences. 



Chemically II could be well placed as a diorite-gabbro, in the 

 use of which term the latter part denotes the type nearest to 

 which the rock approaches. Mineralogically, however, as no 

 trace of a pyroxene exists, the hornblende being all undoubtedly 

 primary and the feldspar an acid labradorite, the use of such a 

 term as diorite-gabbro is not justified. Nor would gabbro-diorite 

 be fully applicable. These two terms should be limited in use to 

 rocks of intermediate minerrdogical composition as well as chemi- 

 cal, else no approach to uniformity can ever be reached. Rock II 

 has differentiated on a line not yet covered by proper names. 

 Hence it will also be designated by the simple term diorite, as I, 

 and of very basic composition. 



Graphic Representation. — All the analyses of dioritic rocks of 

 Table I, except V, have been plotted in the figure, Plate 18. t 



*" Copper-bearing Rooks of Lake Superior." U. S. Geo]. Surv. Monograph V. 

 t Suggested by Dr. A. C. Lawson. 



