Page:The Journal of geology (IA journalofgeology11893univers).pdf/20
consisting of plagioclase and an orthorhombic pyroxene, and therefore corresponding in part to Zirkel's hyphersthenites. The principal difference between the gabbros and diabase was, then, one of structure, while subordinate to this was a difference in mineralogical composition. In his sentences closing the discussion of the gabbros Rosenbusch writes: "Man müsste aber alsdann das Hauptgewicht für die Absonderung der Gabbros nicht auf den eigenthümlich struirten Diallag legen, sondern darauf, dass sie einen pinakoidal spaltbaren klinorhombischen Pyroxen als wesentlichen und daneben einen rhombischen Pyroxenen als accessorischen Gemengtheil enthielten." The distinction here made is evidently a strained one, for quite a number of gabbros were known in which the structure is the typical gabbro structure, while at the same time they are entirely free from rhombic pyroxenes. The new group name "Norites" is borrowed from Esmark and Scheerer, although the rocks described by these geologists are by no means typical of the group. The advantage of the name over "hyphersthenite" is readily appreciated when it is remembered that the rhombic pyroxene of these rocks is not always hyphersthene.
The publication of Rosenbusch's classification of the massive rocks fixed the characteristics of the various types with some degree of scientific accuracy. There was, however, much to be learned concerning the less well known types, and much more to be discovered concerning the relations of the various types to each other.
The work of Judd, referred to above, was the beginning of a severe attack on the wavering line of geologists who still clung to the belief that mineralogical differences alone should determine the class to which a rock should be referred. It would be unprofitable in the present place to mention all of the important articles treating of gabbros and their varieties. It will be sufficient for our purposes to refer briefly only to those papers in which new types of gabbro are described and a little more fully to those which treat of the classification of these rocks.
The existence of true hyphersthenites (norites), of gabbros,