160 J. Nolan — Metamorphic and Intrusive Rocks of Tyrone. 



deposited, must have sunk the underlying metamorphic rocks to 

 such an immense depth that their lowest portions were completely 

 fused and became intrusive. This latter action must have taken 

 place prior to the commencement of the Carboniferous period, as, at 

 Slieve Gallion, the basal beds of that formation rest upon the 

 granite, and are, in fact, almost altogether made up of its debris. 



That metamorphic action continued to affect the rocks of this 

 district at various periods up to that of the Upper Old Ked Sandstone, 

 seems to have been the opinion of General Portlock, and he regards 

 the granite as a result of the metamorphism of rocks of the latter 

 formation. Indeed, if his view be correct, the formation of the 

 granite should be regarded as of post-Carboniferous age, as it is 

 really the lowest beds of that formation — the red sandstones and 

 quartzose conglomerates — that he calls Upper Old Eed Sandstone. 

 The sections he describes at Slieve Gallion, as exhibiting changes 

 from red sandstone through partially altered rocks containing 

 felspar, into granite, have been just referred to, and it has been 

 shown that these appearances are due to the fact, that the sandstones 

 in proximity to the granite are almost altogether made up of its 

 fragments, in many cases, too, of crystals that have undergone but 

 slight attrition, so that the distinction between the crystalline and 

 the derivate rock is often a matter of considerable difficulty. 



On the whole, therefore, there appear to be good grounds to 

 establish the conclusion, that we have in Ireland an intrusive granite 

 of Old Ked Sandstone age. Hitherto most of the granite of this 

 country has been considered to be of older date except that of 

 Mourne and the elvanites of Carlingford, which are post-Carbon- 

 iferous. 1 The discovery of granite of an intermediate period is, 

 however, but what we might expect, as we have no reason to 

 suppose but that it was formed during every period, even at the 

 present day, when, as Professor Jukes remarks — " Granite must be 

 forming now wherever molten rock of the proper chemical com- 

 position is cooling under the requisite physical conditions ; " and we 

 may expect that the further investigation is extended, the better 

 will we be able to assign to their respective ages in the history of 

 our earth, a class of rocks, which, not very long since, were believed 

 to be essentially primeval and antecedent to all others. 



Note. — Since the above was written, Mr. Kinahan's book on the 

 "Geology of Ireland" has appeared, in which the rocks here 

 described as Old Eed Sandstone are classed with the Silurian 

 system. Without expressing any opinion on this question, I may 

 remark that it does not at all alter the geological position of these 

 beds, as they are referred to a later age than any of the generally 

 recognized Upper Silurian rocks — while the era of intrusion of the 

 granite must still be regarded as of post-Silurian and pre-Carbon- 

 iferous age. 



1 Mr. Symes describes an intrusive granite near "Westport as " of older date than 

 the Upper Silurian period," hut there does not seem to be any evidence to fix its 

 age more definitely. See Geol. Survey Memoir to accompany Sheets 83 and 84. 



