THE BASIC MASSIVE ROCKS, ETC. 455 



must be defined. Brauns, 1 in the article referred to in the last 

 footnote, has attempted this correlation. He finds, after review- 

 ing the opinions of various writers on the subject, that " It is not 

 possible to distinguish between diabase and melaphyre on purely 

 petrographical grounds, whether olivine is considered as an 

 essential component of melaphyres, as Rosenbusch holds, or 

 whether it is regarded as unessential in these rocks." In order 

 to construct an exact definition for these three types of rock 

 Brauns is compelled to fall back upon distinctions of age, although 

 Rosenbusch 2 in his last article, in which he refers to this subject, 

 declares it as his opinion that " it requires no great foresight to 

 prophesy that in the not very distant future, this separation [of 

 the effusive rocks into an older and a younger series] will be 

 proven untenable." In spite of the almost certainty that Braun's 

 classification will meet with but little favorable acceptance, it is 

 given here in order to complete the sketch of the history of gab- 

 bros and the related rocks. According to Brauns, the basalts 

 are made to include rocks of this class from recent time to the 

 beginning of the Tertiary age. The limit of separation between 

 the melaphyres and the diabases passes through the productive 

 coal measures ; rocks older than this are regarded as diabases, 

 while the melaphyres extend from the Carboniferous to the Ter- 

 tiary. Each group is divided into varieties, according to struct- 

 ure, and into sub-varieties according to mineralogical composi- 

 tion. A tabular grouping of the principal divisions of the 

 effusive rocks of the composition of diabase follows : 



Granular 

 Porphyritic - 

 Glassy - - 



Paleozoic to Pro- 

 ductive Coal 

 Measures. 



Diabase. 



Diabase-porphyrite. 



Diabase-glass. 



Mesozoic to 

 Tertiary. 



Melaphyre. 



Melaphyre -porphyrite 



Melaphyre-glass. 



Tertiary to 

 Recent. 



Basalt. 



Basalt -porphyrite. 



Basalt-glass. 



It is very evident that the introduction of the diabases among 



1 R. Brauns : lb. 5. Systematik der Diabas, Melaphyr undBasaltgesteine. lb. 

 P- 532. 



2 H. Rosenbusch : Ueber die chemische Beziehungen der Eruptivgesteine. Min. 

 u. Petrog. Mitth. XI, 1890, p. 146. 



