Page:The Journal of geology (IA journalofgeology11893univers).pdf/23

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, consequently they may with good reason be included within the gabbro group. But other peridotites and many of the pyroxenites 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 composition, 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 pyroxenite, 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 contains no diallage. The varieties whose names have reference to