NOMENCLATURE OF KEWEENAW AN IGNEOUS ROCKS 767 



albite and oligoclase are actually andesine-oligoclase, his labradorite 

 is andesine, and his anorthite is chiefly labradorite with some 

 bytownite. 



Irving' followed the practice of Pumpelly, but described about 

 twice as many petrographic varieties. He protested against the 

 practice of basing rock names on any such distinction as that between 

 diallage and augite, but followed the custom, nevertheless, in the 

 main, although he tried to discriminate between diabase and gabbro 

 on the basis of coarseness of crystallization, assigning the name 

 gabbro to the coarser grained varieties. Irving's orthoclase gabbro 

 has been called hornblende gabbro by Wadsworth, and porphyritic 

 gabbro by N. H. Winchell; it is nearly the same as Lane's gabbro 

 aplite; recently it has been called oligoclase gabbro by F. E. Wright.^ 



N. H. Winchell,3 in 1881, described thin sections of dolerite, 

 labradorite rock, hyperite, and gabbro. He made the name " doler- 

 ite" so general in meaning as to include gabbro, diabase, olivine 

 gabbro, olivine diabase, augite andesite, and basalt. His "labra- 

 dorite rock" was called "anorthite rock" by Irving, and is now 

 called plagioclasite (or anorthosite), while his hyperite is now known 

 as norite. 



Wadsworth,'* in 1887, proposed a new classification of the 

 Keweenawan igneous rocks on the basis of the alterations which a 

 given type has undergone. Thus, a gabbro whose augite had altered 

 to hornblende he would call a gabbro, diorite. A peridotite may by 

 alteration become a serpentine or a talc schist; in either case Wads- 

 worth would call it still a peridotite, adding a name to indicate its 

 present condition. Consequently, a rock called, for example, a 

 gabbro by Wadsworth, may belong to any one of a dozen types as 

 commonly recognized. Nevertheless, Wadsworth's names as actually 

 applied in this case may be correlated approximately with the names 

 of other writers, as shown in the table. 



Wadsworth indorsed Irving's protest against using the distinction 



1 R. D. Irving, Geol. Wis., Vol. Ill, pp. 167-206, 1880; Hon. V. U. S. G. S., 1883, 

 and Geol. Wis., Vol, I, p. 340, 1883. 



2 F. E. Wright, Science, Vol. XXVII, p. 892, June, 1908. 



3 N. H. Winchell, Proc. A. A. A. S., Vol. XXX, p. 160, 1881. 

 4M. E. Wadsworth, G N. H. S. Minn., Bull. 2, 1887. 



