434 Scientific Proceedings, Royal Dublin Society. 



The actual epocli of irruption is, therefore, very uncertain, and 

 cannot yet be conclusively established. The same uncertainty 

 also exists about the Arran Grranites and spherulitic felsites. 

 Touching the age of the latter, Allport writes : " So far as is yet 

 known with certainty, they may have been intruded at any time 

 between the close of the Carboniferous and the close of the Ter- 

 tiary Periods. 1 Both at Newcastle and Hilltown the spherulitic 

 rocks break through the Silurian slates, but nothing more definite 

 as to their age can be stated. But at the summit of Slieve Bear- 

 nagh the dyke pierces the Mourne Granite, and may possibly be 

 of Tertiary age, according to Professor Hull. 2 This fact upsets 

 the idea that the dykes are apophyses of the granite, but brings 

 them, on the other hand, into relationship with the trachytes of 

 Co. Antrim. 



This brings us to the question of nomenclature. Spherulitic 

 felsite-porphyry would accord very well with the microscopic 

 characters of the rock ; but the employment of such a name would 

 imply something as to their age. They might, with equal pro- 

 priety, be called quartz-trachytes or Liparites, and they would 

 certainly be placed in the trachytic group, were they definitely 

 known to be of Tertiary age. Allport is of the same opinion as 

 regards the Arran spherulitic rocks, and especially mentions that 

 there is no mineralogical or structural difference between them 

 and the recent trachytes ; there is also, on the other hand, nothing 

 to distinguish them from much older rocks. 3 But, even if we 

 consider the pre-Tertiary age of the rocks established, there seems 

 little reason in pursuing the unphilosophical system of making- 

 geological age,jyer se, a factor in petrographical nomenclature. The 

 older rocks are, of course, more liable to undergo mineral changes 

 during the vicissitudes of their longer history. The less stable 

 minerals will have disappeared, and more stable forms will be 

 substituted. In short, an ancient rock, like a living creature (to 

 quote the words of Professor Bonney), can hardly fail to exhibit 

 signs of old age. Teall, in his series of excellent papers on the 

 Cheviot rocks, has shown that, except for alterations which can 

 only be attributed to the effect of time, some of the porphyrites 



l. c, p. 544. 2 l. c, p. 36 (Memoir). 3 I. c, p. 544. 



