THE BASIC MASSIVE ROCKS, ETC. 449 



gabbro proper in the absence of diallage and orthorhombic 

 pyroxenes. To this variety belong the norite 1 of New York 

 State, the labradorite rock of Labrador, and the "anorthite 

 rock" of Irving 2 from the north shore of Lake Superior. 



But if we are to regard the anorthosites as gabbros in which 

 pyroxene and olivine are wanting, we must pass to the other end 

 of the series and include in the gabbro group those rocks in 

 which plagioclase is wanting, and in which the sole essential 

 components are pyroxene and olivine, or the pyroxenes alone — 

 the peridotites of most authors and the pyroxenites of Williams. 3 

 Judd 4 has shown conclusively that the peridotites of Scotland 

 are but phases of the gabbro with which they are associated, con- 

 sequently they may with good reason be included within the 

 gabbro group. But other peridotites and many of the pyrox- 

 enites must be regarded as distinct rocks. They are the products 

 of the cooling of magmas of an essentially different composition 

 from that of the gabbros, hence their consideration may well be 

 excluded from this history. 



The varieties of gabbro that depend upon mineralogical com 

 position, so far as known, have been carefully described and 

 named by their investigators — the names referring for the most 

 part to the nature of their iron-bearing constituents. These are 

 gabbro and olivine-gabbro, hyperite, norite, peridotite and pyrox- 

 enite, together with the alteration products of the first named, viz.: 

 hornblende, saussurite, orthoclase, and perhaps quartz-gabbro, 5 

 the latter of which is more properly a quartz norite, since it con- 

 tains no diallage. The varieties whose names have reference to 



iCf. F. D. Adams : 1. c, p. 475 and 483. 



2 R. D. Irving: Copper-Bearing Rocks of Lake Superior. Mon. V. U. S. Geo], 

 Survey, p. 438. 



3 G. H. Williams : The non-Feldspathic Intrusive Rocks of Maryland and the 

 course of their Alteration. Amer. Geologist, VI, 1890, p. 95. Not the pyroxenites of 

 the French authors, which are mainly augite gneisses or schistose gabbros. 



4 J. W. Judd : On the Tertiary and older Peridotites of Scotland. Quar. Jour. 

 Geol. Soc, XLI, 1885, p. 357. 



5Cf. U. S.Grant: Note on the Quartz-Bearing Gabbroin Maryland. Johns Hop- 

 kins Univ. Circ. No. 103. 



