224 Grenville A. J. Cole — Igneous Rocks of Stanner. 



among which hornblende is occasionally recognizable ; and quartz 

 often fills the interspaces between the sharply-defined ends of 

 felspar prisms. In other specimens, notably in a coarse variety on 

 the most northern summit, ophitic structure prevails, the felspar 

 being labradorite, according to a determination by Szabo's method 

 made by Mr. J. F. Brooks. The most finely-grained rock of this 

 series appears as a compact grey-green vein above Stanner cliff, 

 and, with its porphyritic plagioclase felspars characteristically cor- 

 roded by the matrix, and its brown clouded patches of augite, at 

 first difficult of recognition, must be classed as an augite-andesite, 

 and, when fresh, must have resembled many Tertiary lava-flows. 

 The whole of the foregoing altered series, from the decomposed 

 (olivine) gabbros to this hemicrystalline vein, come, then, con- 

 veniently under the head of " Diabase." It is, however, this very 

 comprehensiveness, sanctioned by long usage — witness " Diabas- 

 mandelstein" — that makes the name valueless when limited to one 

 rock-species. 



Prof. Judd l has recently pronounced against " Diabase " in favour 

 of " Gabbro " for coarsely-crystalline rocks that can be shown to 

 have contained olivine as well as plagioclase and augite, and has 

 discussed the current terminology from a basic point of view. Since 

 there is a growing tendency to employ "Diabase" for the granitic 

 augite-plagioclase rocks, thereby defining closely what has become 

 a really valuable field-term, I may perhaps venture to give briefly a 

 historic outline of the position. 



In 1818 Alexandre Brongniart 2 put forward the name Diabase 

 for rocks " composee essentiellement d'amphibole hornblende et de 

 felspath compacte," including in this the famous orbicular rock of 

 Corsica. Haiiy 3 subsequently employed Diorite (implying the dis- 

 tinctness of the two constituents) for the same rocks, which he 

 distinguished from syenite by a greater predominance of hornblende ; 

 and this name prevailed so far that Brongniart in 1827 i very grace- 

 fully yielded up his prior term — invented, as he pathetically remarks, 

 by one familiar with ancient and modern languages — lest science 

 should be cumbered with two names for precisely the same thing. 

 He admits diallage and mica among the accessory minerals of the 

 Diorite which he thus helped to establish. 



Diabase therefore ceased to exist, until Hausmann 5 very effectually 

 revived it for the series of rocks containing " hypersthene," labra- 

 dorite, and chlorite, varying in structui-e from highly-crystalline to 

 scoriaceous, which he found associated with his two classes of 

 Diallage-Labradorite and " Hypersthene "-Labradorite rocks. But 

 this " hypersthene " being for the most part only a lustrous augite, 

 analyses revealing its chemical identity with diallage, 6 Diabase 



1 Quart. Journ. Geol. Soc. vol. xlii. pp. 61-2. 



2 " Essai d'une classification des roches," Journ. des Miues, July, 1813. 



3 Traite de Min. 2nde edit, tome iv. p. 536. 



4 Classification des roches homogenes et heterogenes, p. 80. 



5 Ueber die Bildung des Harzgebirges, 1842. 



6 See vom Kath, " Chemiscke Untersuchungen einiger Griinsteine aus Schlesien," 

 Poggend. Annalen, xcv. (1855) p. 545. 



