548 DE. F. H. HATCH ON TEE SPHEEOlD-BEAHINa 



33. On the Sphekoid-beaeing Geanite of Mullaghdeeg, Co. 

 Donegal. By Peedeeick H. Hatch, Ph.D., P.G.S., of the 

 Geological Survey. (Read May 23, 1888.) 



[Communicated by permission of the Director-General of tlie 

 Greological Survey.] 



[Plate XIV.] 



On returning from a recent inspection of the field-work of the 

 Geological Survey in Ireland, Prof. A. Geikie brouojht 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. K. Kilroe, of the Geological 

 Survey of Ireland, in granite at MuUaghderg, Dungloe, Co. Donegal. 

 According to Mr. Kilroe, they occur in a mass, measuring 5 or 6 

 cubic yards, which lies in coarse granite. It consists " of concre- 

 tionary 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 much 

 finer-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 con- 

 cretions ; 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 flesh- 

 tint to a reddish brown. The specimens in my possession have a 

 somewhat loose and crumbly texture, which, if general, would 



* I have examined a section of the rock from this dyke. It is a porphyrite, 

 consisting mainly of a microcrystalline aggregate of felspar and quartz, the 

 latter being subordinate. The sections of the felspar are bounded pai'tly by 

 rectilinear, partly by irregular contours. Some of the grains are striated ; 

 others, on the other hand, shovr no trace of twinning. It is therefore not 

 improbable that orthoclase is associated with the plagioclase. The quartz- 

 grains may be distinguished from the felspars by their greater pellucidity, 

 their more irregular shape, and by the fact that occasionally a uniaxial inter- 

 ference-figure may be obtained. Traces of micropegmatite are not unfrequent. 



Imbedded in this ground-mass are isolated porphyritic crystals of striated 

 felspar, characterized by a marked zonal structure. The extinction-angles 

 reach a rather high value ; and from analogy with other porphyrites of this 

 character, the felspar is probably labradorite. Scattered somewhat sparingly 

 through the section are ragged blades and plates of a green small-angled mica. 

 This mineral shows strong pleochroism : — a=pale yellow; /3 and ■y = dark 

 olive-green. Associated with the mica are scales of chlorite, grains of epidote 

 and occasionally of sphene, iron-ore in isolated granules, and apatite in slender 

 prisms. 



