Geological Society of London. 329 
the district. The ordinary type is a fairly normal biotite-granite ; 
but a variety at Meillionydd shows an exceptional structure, the 
biotite moulding the other constituents in ophitic fashion. The 
granites are intrusive in the Arenig shales. 
(ii.) Gabbro, diorite, etc., in two small patches only. The rock, 
originally a gabbro, passes into diorite, the diallage becoming am- 
phibolized, and the iron-ores disappearing with the production of 
granular sphene. Near a bounding-fault this hornblendic rock 
becomes locally schistose and gneissic. 
(iii.) Diabase, in the centre, forming the mass of Mynydd-y- 
Rhiw, and occurring in dykes and sheets near Sarn. 
(iv.) Hornblende-diabase showing various relations between the 
augite and hornblende. Besides the conversion of the former 
mineral to the latter, a closely similar hornblende has grown as 
an original border to augite-nuclei. The ‘‘ secondary-enlargement ” 
of hornblende-crystals is also exhibited. 
(v.) Hornblende-picrite in several varieties, forming stratiform 
banks to a thickness of 250 feet, and surmounted by hornblende- 
diabase. The two rocks seem to be in close relation to one another, 
and to have been injected as laccolites between the Upper Arenig 
strata near Penarfynydd and Rhiw. 
(vi.) Dolerite-dykes cutting all the other rocks, and probably 
Post-Carboniferous and Pre-Permian. 
With the exception of the last, all these rocks were referred, on 
such evidence as is available, to the Bala age. 
Il.—May 28, 1888.—W. T. Blanford, LL.D., F.R.S., President, 
in the Chair.—The following communications were read :— 
1. “On the Spheroid-bearing Granite of Mullaghderg, Co. Donegal.” 
By Frederick H. Hatch, Ph.D., F.G.S. Communicated with the 
permission of the Director-General of the Geological Survey. 
This paper deals with a remarkable variety of granite which may 
be compared with the well-known orbicular diorite or Napoleonite of 
Corsica. According to Mr. J. R. Kilroe, of the Geological Survey of 
Ireland, who first discovered this interesting rock, the concretionary 
balls occur in close juxtaposition in a mass of granite of 5 or 6 cubic 
yards in size. They have not been found in any other portion of the 
granite area. 
The author first gave a detailed description of the microscopic 
structure of the normal granite. It is a coarse-grained rock, com- 
posed essentially of quartz, orthoclase, microcline, oligoclase, greenish 
hornblende and black mica. Sphene is an accessory constituent. 
Since it contains no white mica, the rock belongs to that subdivision 
of granite which has been termed granitite. A description of the 
spheroidal bodies was then given. The balls are somewhat flattened, 
the greatest diameter being, in one case, 4 inches, the smallest 9. 
Hach ball consists of two distinct parts, a pinkish central portion 
(the nucleus) and a dark-coloured peripheral or zonal portion (the 
shell). The nucleus consists of an irregular granitic aggregate of 
oligoclase felspar with a little interstitial quartz. The peripheral 
