Page:The Journal of geology (IA journalofgeology11893univers).pdf/29

must be defined. Brauns,^[1] in the article referred to in the last footnote, has attempted this correlation. He finds, after reviewing 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 gabbros 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 Tertiary. Each group is divided into varieties, according to structure, and into sub-varieties according to mineralogical composition. A tabular grouping of the principal divisions of the effusive rocks of the composition of diabase follows:

It is very evident that the introduction of the diabases among