102 REPORT 1871. 



The relative and (as far as possible) the actual aoes of these granites still re- 

 mained to be determined, and in the absence of stratified deposits newer than the 

 Lower Silurian in immediate contact with the granites themselves, the authors 

 beUeved that conclusions might be safely arrived at by considerations connected 

 with the basaltic and felspathic dykes by which the rocks had on several occasions 

 been invaded. 



They had arrived at the conclusion that the granite of Mourne was more recent 

 than that of Slieve Croob by a long interval of geological time ; the former being 

 of Upper Palffiozoic, the latter of, perhaps, Mesozoic age. These general conclu- 

 sions were supported by the following considerations. 



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

 porphyry, and sends ofishoots of this rock in the form of dykes into tlje surromid- 

 ing Silurian strata, as may be very clearly determined by several examples in the 

 vicinitv of Newcastle. Hence the authors inferred that the dykes of quartz-por- 

 phyi-y'and felstone which traverse the granite of Slieve Croob might be referred 

 to "the age of the newer granite of Moiu-ne ; and thus the greater antiquity of the 

 Slieve Croob gi-anite might be determined. 



Trap-dykes — Tlie trap-rocks of the district were classed mineralogically as 

 follows: — (a) Quartz-porphyries and highly silicated felstones. (6) Diorites. (c) 

 Basalts or Dolerites of two ages. 



Considered with reference to relative ages of foimation, the following was the 

 order of succession, in the ascending series. 



(1) Older Bdscdts and Dolerite Dykes. — These form by far the most numerous 

 of all the ti'ap-rocks of the district, occurring in gi-eat numbers along the coast 

 south of Newcastle, and amongst the interior mountains, as at Slieve Muck ; they 

 ai'e also unquestionably the oldest of all the trap-rocks of the district. 



Their age, with reference to the granite of Mourne, was placed beyond question 

 by a large number of examples in which these dykes, after traversing the Silurian 

 rocks, are abruptly terminated at the margin of the granite ; they are therefore 

 older than the gi'anite itself*. These older basalts were found to traverse the Silu- 

 rian rocks in well-formed dykes within vertical (or nearly vertical) walls, and are 

 generally fine-grained, of dark gi-een colour, imdistinguishable from those of newer 

 Tertiary age. Sliced specimens showed under the microscope the composition to 

 be augite, triclinic felspar, and titauo-f errite. 



(2) The next in order of age are the quartz-porphyries and felstones, which (as 

 already stated) branch ofi' from the main mass of the Mourne granite, and are un- 

 questionably of the same age as the granite itself, and often strongly resemble it 

 in its more compact form. Dylves of these rocks are also found traversing the 

 older gi-auite of Slieve Croob. They consist of a felspathic base vrith crystals of 

 felspar, gi-ains and crystals of quartz, and sometimes mica or hornblende, as acces- 

 sories, and in small quantities. 



(3) The Diorite djkes are few in number, the finest example occm-ring at Bos- 

 trevor. It consists of a crjstalline gi-anular aggregate of reddish felspar and horn- 

 blende well developed, and traverses the older basaltic dykes ; but is, they beUeve, 

 older than the granite of Momiie. It is therefore referable to some intermediate 

 period. 



(4) Besides the older basaltic dykes, which are cut off" by the granite, tliere are 

 a few which traverse both the Silurian rocks and the gi-anite of ]Moume itself. 

 These are therefore newer than those previously described ; and as they appear to 

 be connected with those which are found traversing the Cretaceous rocks in Co. 

 Antrim, the authors consider them to be of Miocene age. 



In general aspect there is no decided diflference between the older and newer 

 basaltic dykes ; they have all the external appearance of the Tertiary dykes, which 

 aboimd along the margin of the basaltic plateau of Anti-im, and "in the West of 

 Scotland ; and had it not been for their relations with the granite of Mom'ne, they 

 might have all been included in the same categoiy. 



It might have been supposed that microscopical examination would show some 

 * Sir Hichard Griffith has informed one of the authors that he was already aware of this 

 fact, but bad not published his observations. Souio of these dykes are represented on his 

 Geological Map of Ireland. 



