Page:The Journal of geology (IA journalofgeology11893univers).pdf/28

the intrusive^[1] equivalents of the diabases, which he thinks were effusive under water, with the augite porphyrites as their equivalent terrestrial effusives. The conclusions of Loewinson-Lessing are not at all startling in their originality, for the wide separation in origin of the two groups of rocks here discussed has been suspected by petrographers ever since the classification of rock-types based on age, mineralogical composition and structure, gave way to the classification founded on geological relationships. The placing of the diabases with the effusive rocks will probably be looked upon with favor by all petrographers, especially since Professor Rosenbusch^[2] has treated of them as members of this group in his Heidelberg Lectures, and Brauns^[3] has shown that a typical lava flow of a suitable composition may have the diabasic structure developed in it but a few feet below its upper surface.

Lawson,^[4] on the other hand, has shown conclusively that the coarse grained, ophitic diabases, interbedded with the Huronian slates and quartzites on the north shore of Lake Superior, are not effusive, but are intrusive, and that their intrusion between the fragmentals with which they are associated, must have occurred at a time when these were deeply buried under a great thickness of overlying rocks. Consequently these coarse, holocrystalline diabases must be regared as intermediate in their geological relationships, as they are in their structural features between the hypidiomorphic, holocrystalline, plutonic gabbros, and the typically ophitic, hypocrystalline effusive diabases.

But if the hypocrystalline diabases are classed with the effusives, their position with respect to the melaphyres and basalts