Notices of Memoirs — Prof. Sull on the Mourne Mountains, etc. 421 



n.^OK THE Relative Ages of the Granitic, Plutonic, and 

 Volcanic Eocks of the Mourne Mountains and Slieve 

 Ckoob, Co. Down, Ireland.^ 



By Professor Edward Hull, F.E.S. ; and William A. Tkaill, B.A., of the 

 Geological Survey of Ireland. 



AFTER remarking on the bold and interesting physical features 

 of the district, which in some respects resemble those of 

 Arran, and which had already been objects of investigation by 

 Griffith, Berger, and Bryce, the authors observed that there were, 

 as in Arran itself, two varieties of granite. These had been shown 

 by the Rev. Professor Haughton to differ both in composition and 

 origin ; the soda granite of Slieve Croob (consisting of quartz, ortho- 

 clase, and mica) being of metamorphic origin, and the potash 

 gi-anite of Mourne (consisting of quartz, orthoclase, albite, and mica) 

 being irruptive. The relative and (as far as possible) the actual 

 ages of these granites remained to be determined ; and, in the absence 

 of stratified deposits newer than the Lower Silurian in immediate 

 contact with the granite, the authors considered they had approxi- 

 mately determined these points by considerations connected with 

 the basaltic and felstone-porphyry dykes by which the district had, 

 on several occasions, been invaded ; the conclusions thus derived 

 being that the granite of Mourne was newer than that of Slieve 

 Croob by a long interval of geological time, and that while the 

 former was probably of Mesozoic, the latter was of Palseozoic age. 



This distinction might be otherwise expressed thus : that the 

 metamorphic granite of Slieve Croob was formed out of the Lower 

 Silurian Grits and Slates with which it is associated, while the 

 granite of Mourne was forcibly irrupted amongst the Silurian rocks, 

 which now inclose and surmount it in several places. These differ- 

 ences in manner of formation were clearly shown by the effects of 

 the two granites on the surrounding stratified rocks. 



The granite of Mourne at its margin in some places passes into 

 quartziferous porphyry, and sends offshoots of this rock in the form 

 of dykes into the surrounding Silurian strata, as may be very clearly 

 determined by several examples in the vicinity of Newcastle. 

 Hence the authors inferred that the dykes of quartz -porphyry and 

 felstone which traverse the older granite of Slieve Croob might be 

 referred to the age of the newer granite of Mourne. 



Trap DyTces. — The trap rocks of the district were classed minera- 

 logically as follows : — 



(a) Quartz-porphyries and highly silicated felstones ; (h) Diorites ; 

 (c) Basalts or Dolerites of two ages. 



Considered with reference to relative ages of formation, the 

 following was the order of succession ; — 



(1.) Older Basalts and Dolerite DyTces. — These form by far the most 

 numerous of all the trap rocks of the district, occurring in great 

 numbers along the coast south of Newcastle, and amongst the in- 



* Communicated to Section C, British Association, Edinburgh, with the sanction of 

 the Director-General of the Geological Survey. 



