Page:The Journal of geology (IA journalofgeology11893univers).pdf/27

Professor Rosenbusch^[1] clearly appreciated the value of the work on the basic rocks of the Hebrides, for, in the second edition of his Mikroskopische Physiographie, 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 conditions 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 equidimensional. 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 nevertheless 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