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XV. 



ON THE MODE OF OCCURRENCE AND ORIGIN OF THE 

 ORBICULAR GRANITE OF MULLAGHDERG, CO. DONEGAL. 



By GRENVILLE A. J. COLE, M.R.I.A., F.G.S., 

 Professor of Geology in the Royal College of Science for Ireland. 



(Plates IV-V.) 



Head Fehuuary 23, 1915. Published, with additions, October 26, 1916. 



I. The Occurrence in the Field. 



About 1887, Mr. J. R. Kilroe, of the Geological Survey of Ireland, observed 

 " concretionary balls " in the granite of the townland of Mullaghderg, west 

 of Crolly Bridge, in the county of Donegal. The locality lies on an inlet of 

 Inishfree Bay, close to where the name Gleudowan appears on Sheet 9 of 

 the 1-inch map of the Ordnance Survey, published in 1858, and in revised 

 form in 1905. Mr. Kilroe's specimens were examined petrographically by 

 Dr. F. H. Hatch, 1 and the occurrence is described by the finder, with com- 

 ments by Dr. J. S. Hyland, in the Memoir of the Geological Survey of 

 Ireland to Sheets 3, 4, &c, issued in 1891. ' 



Since that time, the somewhat remote locality has been visited by Miss 

 M. K. Andrews of Belfast, who has very kindly allowed me to use the whole 

 of the material collected by her, and her fine series of thin slices cut from 

 the spherulites and the associated rocks ; by Mr. R. Bell, at the instance of 

 Mr. F. N. Ashcroft, who wished to present a specimen of the rock to the 

 British Museum ; by Mr. R. "Welch, one of whose excellent photographs is 

 reproduced in the present paper ; and by Mr. T. Hallissy and myself, during 

 a recent official mapping of the district on the scale of six inches to one 

 mile. The Geological Survey obtained, as the result of this re-examination, a 

 number of large blocks from the orbicular mass, some of which have been 

 sawn and polished, so as to display the relations and structures of the 

 spherulites (Plates IV and V). 



In the field, this orbicular mass is seen to be a specialised and compara- 

 tively small portion of the typical red granite of the district. This granite 



1 " Oa the Spheroid-bearing Granite of Mullaghderg," Quart. Journ. Geol. Soc. 

 London, vol. xliv (1888), p. 548. 



2 Mem. to Sheets 3, 4, &c, pp. 80 and 137. 



SOIENT. PKOC. R.D.S., VOL, XV., NO. XV. 2 A 



