132 



account of the manner in which those in common use are now employed 

 by the more important writers. 



Gabbro. This is an old Italian name said to have been originally applied 

 to a serpentinous rock containing diallage. It was introduced into general 

 geological literature by L. VON Burn, ZIRKEL defined it (l> as a coarsely 

 crystalline granular rock, composed of labradorite or saussurite and diallage 

 or smaragdite (variety of hornblende). It is defined by ROSEN BUSCH (2) as 

 a pre-tertiarv rock, composed of plagioclase and diallage ; and is divided 

 into two groups according to the presence or absence of oliviiie. The 

 important question which is at present undetermined, is whether diallage should 

 be regarded as an essential constituent of gabbro. STRENG/ S) JuDD, (4) and 

 others have shown that augite occasionally replaces diallage in rocks that have 

 been unhesitatingly referred to the gabbros. Moreover, the latter observer 

 has shown that one and the same crystalline individual may be augite in one 

 part, and diallage in another. He proposes, therefore, that the term gabbro 

 should be applied generally to the coarse-grained granitic modification of the 

 basic magma, irrespective of the nature of the monoclinic pyroxene. In this 

 sense it will be employed in the present work. In addition to the principal 

 constituents, there are others which may be regarded as accessory, and these 

 may again be subdivided into original and secondary. Amongst the original 

 accessories may be mentioned olivine, a rhombic pyroxene, and probably 

 hornblende ; amongst the secondary accessories must be placed " compact " 

 hornblende, as well as the uralitic and actinolitic modifications, zoisite, 

 epidote, albite, and quart/. The secondary accessories mentioned above are 

 especially characteristic of rocks that have been affected by contact and regional 

 metamorphism. They mark a certain stage in that change which may give 

 rise to the formation of hornblende-schist, and become of great importance 

 for the purposes of indicating varietal distinctions. We thus speak of 

 saussuritic, hornblendic, uralitic, and actinolitic gabbros. There are also 

 secondary accessories, (e.g. zeolites, chlorite, serpentine, &c.,) due to weather- 

 ing ; and others, (e.g. magnetite,) due to the process for which Professor 

 JUDD has proposed the term " schillerisation." 



Eupliotide. This name was introduced by HAUY for a rock w r hich we 

 should now designate as saussuritic gabbro. It is employed by Messrs. 

 FOUQUE and LEVY and by French petrographers generally, for a gabbro of 

 Tertiary age. In this sense there is of course no distinction in composition 

 or texture between gabbro and euphotide. Professor BONXEY (5) proposes 

 that, if used at all, it should be employed in its original sense ; that is for an 

 altered gabbro. 



(1) Lehrbuch der Petrographie, 1866, Vol. II., p. 10. 



(2) Die Massige Gesteine (1st edition), p. 459. 



(3) N.J., 1862, p. 943. 



(4) Q.J.G.S. Vol. XLIL, 1886, p. 61. 



(5) Q.J.G.S. Vol. XLL, 1885. Proc. p. 70. 



