454 THE JOURNAL OF GEOLOGY. 



the intrusive 1 equivalents of the diabases, which he thinks were 

 effusive under water, with the augite porphyrites as their equi- 

 valent terrestrial effusives. The conclusions of Loewinson-Les- 

 sing 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 struc- 

 ture, gave way to the classification founded on geological rela- 

 tionships. 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, ^nterbedded 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 regarded as intermediate in their geological 

 relationships, as they are in their structural features between the 

 hypidiomorphic, holocrystalline, plutonic gabbros, and the typi- 

 cally ophitic, hypocrystalline effusive diabases. 



But if the hypocrystalline diabases are classed with" the effu- 

 sives, their position with respect to the melaphyres and basalts 



1 F. Loewinson-Lessing : Quelques considerations genetiques sur les diabases, les 

 gabbros et les diorites. Bull. d. 1. Soc. Beige, de Geol. etc., II, 1888, p. 82. 



2 Cf. Zeits. d. deutsch. geol. Ges. XLI, 1890, p. 533. 



3 R. Brauns: Mineralien und Gesteine unf dem hessischem Hinterland II, 3, 

 Diabas mit geflossener Oberflache (Strick oder Gekroselave) von Quotshausen. Zeits. 

 d. deutsch. geol. Ges. XLI, 1890, p. 491. 



4 A. C. Lawson: The Laccolitic sills of the northwest coast of Lake Superior, 

 Geol. and Nat. Hist. Surv. of Minn. Bull. No. 8, 1893, p. 24. 



