450 THE JOURNAL OF GEOLOGY. 



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 diffi- 

 culty of distinguishing positively anorthite from the other plagio- 

 clases. Since the discovery by Tschermak that the plagioclases 

 form a series of isomorphous compounds, the value of the dis- 

 tinction 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 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 dia- 

 bases by sharper lines than those based merely on mineralog- 

 ical 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 recog- 

 nition of the fact that the value of the pinacoidal parting of 

 diallage is not of great importance for the purpose of rock classi- 

 fication. 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 significa- 

 tion 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 



1 For a discussion of the eukrites see J. Roth : Allgemeine und Chemische Geol- 

 ogy, II, 1883, p. 200. 



2 Allegemeine und Chemische Geologie, II, p. 185. 



