THE BA SIC MA SSIVE R O CKS, ETC. 453 



Professor Rosenbusch 1 clearly appreciated the value of the 

 work on the basic rocks of the Hebrides, for, in the second edi- 

 tion of his Mikroskopische Physiographic, he defines the gabbros 

 as hypidiomorphically granular plutonic rocks, -consisting of a 

 basic plagioclase, diallage, or a pyroxene resembling diallage, 

 rhombic pyroxenes and often olivine. The important feature in 

 this definition is the characterization of the gabbros as plutonic 

 rocks. The diallage no longer defines the gabbro. The condi- 

 tions which determined the characteristic structure of the rock at 

 the same time produced the diallagic structure in its pyroxenic 

 constituent. The structure of the typical gabbros, as defined by 

 Rosenbusch, is granular, with the components all equidimen- 

 sional. Notwithstanding the fact that some plutonic rocks of 

 this class seem to lack the granitic structure, it remains true that 

 the typical gabbro is well described by this definition. 



When, however, we seek to separate the gabbros from the 

 diabases we are met at the outset with the same difficulties that 

 have always stood in the way of. an exact separation of these two 

 rocks. Rosenbusch 2 describes the diabases as possessing some 

 of the features of plutonic rocks, while at the same time they 

 possess other features that are eminently characteristic of rocks 

 that have flowed out upon the surface of the earth. He never- 

 theless includes them with the plutonic rocks, stating, however, at 

 the same time that they occur principally as dykes and interbedded 

 flows ; are more frequently interstratified with schists than are 

 any other plutonic rocks ; and that their predominant structure 

 is the ophitic. That there is a fundamental difference between 

 the two rocks is shown by the fact that the typicial gabbro can 

 not be traced into porphyritic or hyprocrystalline varieties, nor is 

 it ever accompanied by tufas. Whereas the diabases are often 

 porphyritic, and are not infrequently associated with diabasic 

 tufas. A consideration of these phenomena, together with the 

 great differences in the structures of the typical gabbros and 

 diabases, have led Loewinson-Lessing to regard the gabbros as 



'Mikroskopische Physiographic, der Massigen Gesteine, 2, Auf. 1887, p. 132. 

 2 Mikroskopische Physiographic, 2 Auf. II, pp. 174 and 195. 



