lU 



Proceedings of the Roi/nJ Iri-f/i Acnilrmn. 



sketches and descriptions, aided by two small specimens, made it fairly clear 

 that some form of spheroidal weathering had taken place on a jointed surface 

 of igneous rock; hut it seemed well worth while that the mass should 

 be examined geologically. It appeared, indeed, quite possible that human 

 agency might in time be invoked to account for the singular nature of the 

 markings. 



During a ^^sit to northern Ireland on behalf of the Geological Sur\ey 

 in Octol)er, 1909, I was so fortunate as to find the Admiralty surveying 

 vessel still in Lough Swilly, ofl' P.athmullan. Captain Boyle Somerville very 

 kindly guided me across the hills to the Picture-Eock, which lies one mile 

 north-east of Glenalla House, and one-third of a mile north-east of Lough 

 Eogan, on the south-western spurs of Croaghan Hill. It is formed by one 

 of the steeply tilled intrusive sheets in the Dalradian shales and sandstones ; 

 and its dip- slope faces approximately south-east. The dip is about 50°. 



r<S}^^^J-3^^^ 







Fig. 1. — Stmctures Mvn on the cxpowd (tec of the Picture-Rock, from a photograph. AI)Out 



one-fortielh natural aize. 



A large part, but by no means the whole, of the exposed face shows the 

 " blistered " structure noted by Mr. G. H. Kinahan in the Memoir, and on his 

 manuscript map in the office of the Sun'ey. Grey lichen-covered spheroids 

 project from it, out of deep, roughly rectangular, box -like hollows. Between 

 the " boxes," the rock rises to the same general level as the protuberant face 

 of the spheroids ; and the walls of the compartments into which the rock 

 is thus divided clearly depend upon an original structure of two series of 

 joints crossing at right angles. The spheroids depend on the characteiistic 

 onion-like jointing of basic rocks, which has arisen within each box-like 

 compartment, just as it arises within the drums of basaltic columns. 



