442 THE JOURNAL OF GEOLOGY. 



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 con- 

 sisting "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 class- 

 ification 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 micro- 

 scopical 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 plagio- 

 clase is usually labradorite. It usually contains fluid inclusions 

 and numerous little dark needles and prisms arranged in a defi- 

 nite 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 crys- 

 tals and no unindividualized ground mass. 



The group of hyphersthenites had by this time become almost 

 depleted of its members. Most of the hyphersthenites 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 hyper- 

 sthenic group. These rocks were described as having the 

 structure of gabbros, while at the same time they contain but 



1 F. Zirkel : Mikroscopische Beschaffenheit der Mineralien und Gesteine, 

 Leipzig, 1873. 



