GABBEOS ETC. IN SCOTLAND AND IRELAND. 61 



It will be seen, then, that these Hebridean rocks have come to 

 occupy a crucial position in the controversy which has arisen con- 

 cerning the classification and nomenclature of igneous rock-masses. 

 If they are truly gabbros, as maintained by Zirkel, Yon Lasaulx, and 

 the geologists of this country generally, then it must be admitted 

 that the principle of the classification of rocks by their geological 

 age, having been found inapplicable in this case, has received a 

 severe, and, indeed, a fatal shock. 



Those who advocate a purely mineralogical classification of rocks 

 have always insisted that the characteristic mineral of gabbro is the 

 diallage. But so long ago as 1862 it was shown by Strong that in 

 many of the gabbros of the Harz we find the diallage replaced by 

 ordinary augite *. In following Zirkel's classification of the Scottish 

 rocks as gabbros, in 1874, I pointed out that the pyroxene was 

 sometimes in the form of true augite and sometimes occurred as 

 diallage t. The same mutual replacement of augite and diallage I 

 have found to occur in all gabbros which 1 have examined, in which 

 the alteration of the rock had not gone so far as to mask the true 

 character of the pyroxenic constituent. In my paper on the peri- 

 dotites it is shown that the diallage of the Scottish rock has in all 

 cases been produced from ordinary augite by the process for which 

 the name of " schillerization " is proposed. 



By some authors the sole distinction which has been made between 

 the gabbros and the diabases has been that the former contain 

 diallage and the latter ordinary augite. Hence it may be argued 

 that if the distinction between these two mineral varieties be given 

 up as a basis of rock-classification, the name of diabase has as good 

 a right to be adopted as that of gabbro. But against this contention 

 I would point out that the use of the old Italian name of " gabbro " 

 for the class of rocks in question was proposed by Yon Buch s^s, long 

 ago as 1810 J ; that from the first it seems to have been applied to 

 highly crystalline rocks, as indicated by the synonyms " granitone " 

 and " granite di gabbro ;" and that its use in that sense is now 

 almost universal, not only in this country, but in Germany, France, 

 Italy, and North America. The term " diabase," on the other hand, 

 though originally proposed by Alexander Brongniart, was only 

 restricted to pyroxenic rocks by Hausmann in 1814 § ; many authors 

 too, down to quite recent times, have employed " diabase " as a 

 name for rocks which are not perfectly holocrystalline, but contain 

 a considerable proportion of unindividualizcd ground-mass. If, then, 

 it becomes a question between the use of the terms gabbro and 

 diabase for the highly crystalline representatives of the basic scries 

 of rocks, I think there cannot be the slightest doubt that the verdict 

 should be given in favour of gabbro, both on the ground of XDriority 

 and on that of the consistent use of the term by authors in the 

 past. 



* Neues Jabrb. fiir Min. &c. 1862, p. 943. 

 t Quart. Jouni. Geol. Soc. vol xxx. (1874) p. 237. 

 t Magaz. der Ges. naturf. Freunde zu Berlin, aoI iv. p. 128. 

 § * Ueber die Bildung des Harzgebirges.' 



