Proceedings of the Royal Irish A cademy. 



II. — The Case of Careickgollogan. 



The quartzite cone of Canickgollogaii, with its associated green shales, 

 similar to those that include Oldhamia at Bray Head, appears on the map 

 of the Geological Survey (sheet 121) as an inlier among Ordovician rocks. 

 Jukes and Wyley'* say that it is " surrounded [bounded ?J on one side by 

 mica schist, and on the other by black slate, both of which we believe to be 

 Silurian." This conclusion resulted from the preliminary survey, on which the 

 map issued in 1855 was based. A "historic series" of proofs and successive 

 standards for each one-inch sheet has now been arranged in the office of the 

 Geological Survey, and it has been possible to trace the various developments 

 that led to the revision of sheet 121 in 1865. It has long been known that 

 such revisions occurred in many sheets issued by the Irish Survey, without 

 any change being made in the original engraved date. The proofs dated in 

 manuscript, and the copies of sheet 121 issued after 1865, are now the only 

 clue to the sequence of various copies of the MS. six-inch Geological sheet, 

 Co. Dublin, 26, which includes the CarrickgoUogan area. 



By collating these documents, some facts of interest come to light. 

 A. Wyley mapped the ground in the first instance in 1853, and recognized 

 the " Cambrian look " of the green shales along the south side of the quartzite 

 boss. On its north side, and south of the second quartzite band, he noted 

 that they were "not at all micaceous." This is evidently an expression of 

 his surprise that they were not altered by the granite. On another copy of 

 this six-inch sheet, Wyley (or Du Noyer ?) adds that the shales may be 

 " protected by the quartzite bed." A fine-grained granite dyke occurs in the 

 mica-schists close at hand, and the margin of the main granite of Ballycorus 

 and the Scalp lies only 700 yards (640 m.) away. 



When Jukes approved and passed the reduction of the lines to the one- 

 inch scale on February 7th, 1854, the long thin band of shale on the south 

 was inserted as Cambrian, and the whole elliptical Cambrian area was shown 

 as cut off at its south-west end by a fault extending from the district of the 

 Great Sugarloaf. ISTo authority for this fault exists on any six-inch MS. map, 

 though at CarrickgoUogan it is quite a local probability. G. V. Du Noyer 

 examined the ground in detail in 1860, and evidently was puzzled by the 

 apparent superposition of the block of CarrickgoUogan on Silurian schists and 

 slates. The notes on his revised map show that he found " no evidence to 

 prove the quartz rock of CarrickgoUogan ' Cambrian,' " and he states that it 

 rests " in a hollow of the bedded Silurian slates." M'Henry does not seem 



Op cit. , Journ. Geol. Soc. Dublin, vol. vi, p. 40 (1856) 



