Page:The Journal of geology (IA journalofgeology11893univers).pdf/16

heretofore described as containing hypersthene, have none of this mineral in their composition. He divided the gabbros into those containing olivine and those without this constituent, and from the latter separated a group which he called saussurite gabbros, recognizing at the same time, however, that saussurite is an alteration product of labradorite. He described it as consisting "of small crystal needles, prisms and grains, which are colorless or light-green, and are scattered irregularly in a ground mass with the appearance of a colorless glass, which often forms clear patches in the saussurite" (p. 52).

Six years after Rose's description of the Neurode gabbro, and seven years after the appearance of Zirkel's masterly classification of rocks based almost entirely upon their macroscopic properties, the latter geologist was enabled to issue a second volume containing a classification of rocks based on the microscopical characters. In this volume^[1] he defines the gabbros as granitic in structure, and consisting principally of plagioclase and diallage, usually with the addition of olivine. The plagioclase is usually labradorite. It usually contains fluid inclusions and numerous little dark needles and prisms arranged in a definite order. The diallage is filled with small brown plates and the olivine is characterized by thousands of fantastically shaped hair-like bodies. The structure of genuine gabbros is described as coarsely or finely granular. They contain no porphyritic crystals and no unindividualized ground mass.

The group of hyphersthenites had by this time become almost depleted of its members. Most of the hypersthenites had been found to be diallagites, in the sense of Des Cloizeaux, so that but four undoubted occurrences of this rock were left to be included by Zirkel in the group. On the other hand, the number of "forellensteins" had increased to such a degree that a group was formed of the same classificatory value as that of the hypersthenite group. These rocks were described as having the structure of gabbros, while at the same time they contain but