96 PROF. J. W. JTJDD ON TERTIARY 



he thoroughly agreed. He cordially endorsed the high opinion 

 expressed by the author of the work done by Messrs. Hague and 

 Iddings upon Transatlantic rocks. He remarked that lavas of totally 

 distinct characters are poured out from the same vent, so that the 

 use of the term " petrographic province " seemed to be of rather 

 doubtful propriety. Glomero-porphyritic structure, he thought, 

 might perhaps be due to the breaking up of crystals, as occasionally 

 seen in glassy rocks. The cavernous structure of some crystals 

 was probably not due to corrosive action in the rocks. Palagonite 

 had been discussed some years ago, and its nature appeared to be 

 doubtful. Pelspar crystals were apparently not always the minerals 

 first formed in rocks of a doleritic or andesitic type. 



Mr. J. Starkie Gardner said that he had been working for some 

 four or five years upon the question of the age of the leaf-beds 

 associated with the basalts in Ireland and the Western Islands of 

 Scotland. The conclusion at which he was arriving was that they 

 extended from pre-Eocene (although not Cretaceous) to Middle 

 Eocene times. 



Prof. Boyd Dawkins remarked that the microscopical study of 

 rocks seemed to have led to much the same conclusions as the 

 earlier and rougher modes of macroscopic investigation- The rela- 

 tions of structure to heat and pressure in presence of water had 

 been made out before the more delicate methods of modern petro- 

 logical research had revealed the minuter details. The old classi- 

 fication of the rocks in its main outlines still holds good. 



Mr. Teall said that all petrographers would thank Prof. Judd 

 for his detailed descriptions of the Tertiary igneous rocks of the 

 west of Scotland. The hesitation which many workers had shown 

 in accepting his views was probably due in a great measure to 

 the absence of detailed petrographical descriptions. He agreed 

 with the author that there was a distribution of igneous rocks 

 in space and time which could only be described in a satis- 

 factory manner when a classification based solely on the chemical, 

 mineralogical, and physical characters of rocks was established. 

 The introduction of geological age as a primary factor in classi- 

 fication obscured natural relationships, implied differences which 

 did not exist, and rendered impossible the accurate description 

 of the distribution in time of many important rock-types. In 

 limited areas there were differences which could be definitely con- 

 nected with geological age. The ophitie texture, for instance, was 

 especially characteristic of the pre-Tertiary basic igneous rocks of 

 Germany ; and absent, so far as the speaker was aware, from the 

 corresponding rocks of Tertiary age in the same area. It would be 

 a mistake, however, to regard this texture as a special characteristic 

 of pre-Tertiary rocks all the world over, or as of the smallest use in 

 fixing the age of a rock from any new locality ; for it was now 

 known to be a special feature of the dolerites of the north of 

 Ireland, the west of Scotland, and Iceland. 



The Author, in reply, said that the term " gabbro " has been so 

 extensively employed that it cannot well be dropped. Mr. Eutley, 



