On the Spheroid-bearing Granite of Mullaghderg, Co. Donegal

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Abstract
On returning from a recent inspection of the field-work of the Geological Survey in Ireland, Prof. A. Geikie brought with him some remarkable specimens of spheroidal concretions, which, on account of their interesting structure, he thought worthy of detailed examination and description. For this purpose they were entrusted to me, and the present paper records my observations. These bodies were found by Mr. J. R. Kilroe, of the Geological Survey of Ireland, in granite at Mullaghderg, Dungloe, Co. Donegal. According to Mr. Kilroe, they occur in a mass, measurig 5 or 6 cubic yards, which lies in coarse granite. It consists “of concretionary balls, varying in size up to 6 inches in diameter. The balls are usually flattened and lie almost contiguously, the interstices being filled by granite, which is similar in character to, though muchfiner-grained than, that surrounding the entire mass.” In a letter I have since received from Mr. Kilroe he informs me that dykes are numerous in the locality where the spheroids occur. One of these passes quite close to the mass containing the concretions; but since the latter are confined to a small space and do not follow the dyke, I see no reason for connecting their formation with its intrusion. The Normal Granite.—Of the granite referred to above, Mr. Kilroe was kind enough to furnish me with specimens. It is a coarse-grained rock, varying in colour from a pale fleshtint to a reddish brown. The specimens in my possession have a somewhat loose and crumbly

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