422 Notices of Memoirs — Prof. Hull on the Mourne Mountains, etc. 



terior of the Mourne mountains, as at Slieve Muck and Pigeon Eock 

 Mountain and others. 



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, evincing a higher antiquity than the 

 granite itself.^ 



These older basalts were found to traverse the Silurian rocks in 

 well-formed dykes within vertical (or nearly vertical) walls, and 

 are generally undistinguishable from those of newer Tertiary age. 

 Sliced specimens showed under the microscope the coniposition to be 

 augite, triclinic felspar, and titano-ferrite. 



(2.) The next in order of age are the quartz porphyries and fel- 

 stones, which (as already stated) branch off from the main mass of 

 the Mourne granite, and are unquestionably of the same age as the 

 granite itself, and often strongly resemble it in its more compact form. 



Dykes of these rocks are also found traversing the older granite 

 of Slieve Croob. They consist of a felspathic base, with crystals of 

 felspar grains and crystals of quartz, and sometimes mica or horn- 

 blende as accessories in small quantities. 



(3.) The diorite dykes are few in number : the finest example 

 occurring at Eostrevor, where a large dyke traverses the older 

 basalt dykes contained in the Silurian beds. It consists of a crystal- 

 line granular aggregate of reddish felspar and hornblende well 

 developed. 



(4.) Besides the older basaltic dykes, which are cut off by the 

 granite, there are a few which traverse both the Silurian rocks and 

 the granite of Mourne itself. These are, therefore, newer than those 

 previously described. 



In general aspect there is no decided difference between the older 

 and newer basaltic dykes ; they have all the external appearance of 

 the Tertiary dykes which abound along the margin of the basaltic 

 plateau of Antrim, and in the west of Scotland ; and had it not been 

 for their different relations to the Mourne granite, they might have 

 all been included in the same category. 



It might have been supposed that microscopical examination would 

 show some distinction in the basalts of these geological ages, but 

 recent investigations by Zirkel, D. Forbes, AUport, and others, tend 

 to show that there is no criterion of age amongst the constituents of 

 basalt, dolerite, or melaphyre ; and the presence of olivine — once 

 supposed to be distinctive of Tertiary basalts — has been detected 

 amongst those even of Carboniferous age.^ Nor can the bearing of 

 these dykes form any basis of distinction, as in the Moume district 

 the older basaltic dykes run in all directions ; the easterly and 

 westerly dykes, however, appear to cut those bearing North and 

 South. 



Age of the older basalts. — The geological age of these older 



' Sir Richard Griffith has informed one of the authors that he was already aware 

 of this fact, but had not published his observations. 

 ^ Mr. S. Allport, Geological Magazine, Vol. VI., pp. 115 and 159. 



