446 THE JOURNAL OF GEOLOGY. 



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 dis- 

 cussion of the gabbros Rosenbusch writes : " Man miisste aber 

 alsdann das Hauptgewicht fur die Absonderung der Gabbros 

 nicht auf den eigenthumlich struirten Diallag legen, sondern 

 darauf, dass sie einen pinakoidal spaltbaren klinorhombischen 

 Pyroxen als wesentlichen und daneben einen rhombischen Pyrox- 

 en 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 appre- 

 ciated 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 deter- 

 mine 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 suf- 

 ficient 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, 



