Page:The Journal of geology (IA journalofgeology11893univers).pdf/24

the feldspathic component are the orthoclase-gabbro of Irving and the eukrites^[1] of the older authors. The latter name was proposed to designate rocks whose feldspar is anorthite. It never received a very wide application owing partly to the difficulty of distinguishing positively anorthite from the other plagioclases. Since the discovery by Tschermak that the plagioclases form a series of isomorphous compounds, the value of the distinction recognized by the name has disappeared and the name itself has fallen into disuse.

In addition to these there are two other varieties that seem to be sufficiently well characterized to deserve some special names. One of these, the anorthosite, consists exclusively of gabbroitic plagioclase and the other "forellenstein" contains olivine and plagioclase.

During the past few years nearly all the work on the gabbros has tended toward the separation of these rocks from the diabases by sharper lines than those based merely on mineralogical distinctions. All those who had attempted to separate the two groups by the methods in use had failed, and some had thought it well to include the two in one group. The views of the earlier petrographers on this subject have been referred to. Later petrographers have accorded with these in their recognition of the fact that the value of the pinacoidal parting of diallage is not of great importance for the purpose of rock classification. The discovery of Judd, referred to above, produced a marked effect on the work of those who followed him in the same field.

In 1883 J. Roth^[2] declared that the position of the gabbros with respect to the diabases depends upon the signification given to diallage. If we regard it as an altered augite with a pinacoidal parting produced by twinning it is found, as Rosenbusch has already stated, that the parting may occur in the pyroxene of some rocks without the presence of