58 PEOF. J. W. JUDD ON TEETIART 



of the age of the Upper Greensand and the Chalk. It was proved 

 that these Cretaceous rocks, which include strata younger than the 

 zone of Belemnitella mucronata, are overlain unconformably by the 

 great sheets of basaltic rock*. 



In the north of Ireland the proofs of the Tertiary age of these 

 igneous rocks is equall}' clear and convincing. Everywhere they 

 are seen overlying the eroded surfaces of the " white limestone " 

 (Chalk), which includes beds certainly as high in the series as the 

 zone of Belemnitella mucronata. They contain, moreover, inter- 

 bedded deposits like those of Mull, which have yielded numerous 

 remains of a Tertiary flora. 



With respect to the exact portion of the Tertiary period to which 

 the plants obtained from the strata intercalated with the basalts of 

 Scotland and Ireland should be referred, some misconception seems 

 to have existed ; but we may confidently hope that the persevering 

 researches which are now being undertaken by Mr. J. Starkie 

 Gardner will throw much new light upon this interesting question. 



It has beeii shown, by both Dr. A. Geikie and myself, that the 

 great basaltic plateaux are broken through by intrusive masses of 

 still later datef. I have found no evidence of any volcanic out- 

 bursts having taken place in the British area at anj^thing approach- 

 ing to recent times. In Iceland, however, where the general mass 

 of the strata, judging from the plant-remains and other fossils 

 contained in interbedded strata, must be of Tertiary age, the volcanic 

 action has continued on a grand scale down to the present time, 

 though there are not wanting evidences that this volcanic action 

 may now be approaching the stage of final extinction. 



The facts which I have now passed in review warrant the assertion 

 that the igiteous masses of this Brito-Icelandic petrographical 

 province — from the most acid to the most basic in composition, and 

 including representatives of every variety of structure, from the 

 most highly crystalline to perfectly vitreous types — are all of post- 

 Cretaceous age, and must have been erupted during different portions 

 of the Tertiary epoch, coming in some cases quite down to recent 

 times. 



IV. IS'oMElSrCLATUEE OF THE EoCKS. 



There is fortunately no difference of opinion among petrographers 

 as to the name which should be applied to the imperfectly crys- 

 tallized types of the rocks we are considering ; all authors are 

 agreed that they should be referred to the " basalts." The great 

 mass of the lava-streams that have built up the plateaux of Antrim 

 and the "Western Isles of Scotland belong to the felspar- or plagio- 

 clase-basalts ; * and, as I shall show in the sequel, to the same type 

 of those rocks which is so largely represented in Iceland. As in 

 Iceland, too, the successive outflows of basaltic lavas were occasionally 



* Quart. Journ. Geol. Soe. vol. xxxiv. (1878) pp. 728-737. 



t Ibid. vol. xxvii. (1871) p. 294 ; ihid, vol. xxx. (1874) pp. 260-267. 



