60 PKOP. J. W. JUDD O^sT TERTIAEY 



exhibit all the characters which petrographers have agreed to be 

 distinctive of gabbros, they ought to be called by that name and 

 no other*. 



The determination of these rocks made by Professor Zirkel in 1871 

 was very strongly supported by the researches of Professor von Lasaulx 

 in 1878 1. This author's observations were made upon a series of 

 specimens from the Carlingford Mountains. He pointed out that 

 these rocks have the closest analogy with the olivine-gabbros of 

 Mull and Skye ; that, like those rocks, they contain plagioclase 

 felspar (anorthite or a variety approaching that species), diallage, 

 and olivine, with some magnetite of secondary origin. Professor von 

 Lasaulx also found that the minerals of the Carlingford rock contain 

 both liquid and solid enclosures. 



Both Professor Zirkel and Professor von Lasaulx are agreed that 

 the characters of these rocks are those of true gabbros, and they 

 insist on their exact similarity in mineralogical constitution and 

 microscopic structure with typical gabbros, like those of Silesia, 

 jSTorway, and Northern Italy. It is, I think, certain that, had not 

 the Tertiary age of these rocks been demonstrated, no petrographer 

 would have dreamed of removing them from the class of the 

 gabbros. 



In the year 1877 Professor Eosenbusch published his very valuable 

 work, ' Mikroskopische Physiographic der massigen Gesteine,' in 

 which he unfortunately assumed that fundamental distinctions exist 

 between the igneous rocks which were erupted before the Tertiary 

 period and those which have made their appearance since the com- 

 mencement of that epoch. Referring to these and similar rocks, 

 he writes : — " If we carry out consistently the nomenclature em- 

 ployed in this book, then Zirkel's designation of the Hebridean rocks 

 as gabbros is incorrect. These rocks would bear the same relation 

 to basalt as olivine-gabbro to olivine-diabase, and would therefore 

 have to be characterized as diallage-basalts of granular structure "J. 



The Prench geologists have, on the other hand, proposed to restrict 

 the term gabbro to the older crystalline basic rocks, and for the 

 similar rocks of more modern date to employ Haiiy's name " eupho- 

 tide." But M. de La^Dparent, in applying this name to the rocks of 

 Skye and Mull as well as to those of Liguria, makes the express 

 admission that " there are modern euphotides which are not distin- 

 guished in any way from the ancient gabbros " §. 



The geologists and petrologists of this country have always 

 refused to recognize the geological age of a rock as a character upon 

 which its classificatory position and nomenclature ought to be based. 

 They regard such a proceeding as both inexpedient and impracticable 

 — inexpedient, inasmuch as it prejudges the question of the distri- 

 bution of rocks in time ; and impracticable, in that there are many 

 eruptive rock-masses of which it is quite impossible to determine the 

 geological age. 



* Zeitschr. d. d. G-eol. Gesell. xxiii. (1871) pp. 58 & 93. 



t Tschermak's Min. und Petr. Mittb. vol. i. (1878) pp. 426-433. 



X Loc. cit. p. 476. § Traite de Geologie, 2^^ ed. (1885) p. 634. 



