GoL'E—On Composite Gneisses in Boylayh, West Donegal. 209 



along the foliation-planes of the mica-schist. The latter rock contains, 

 in addition to the usual pale biaxial mica, a green-brown biotite, 

 probably developed as a product of the igneous contact.' A specimen 

 measuring 7 cm. by 8 cm. by 2 cm., from west of the farm of Ard- 

 lougher, and formed of clearly defined sheets of muscovite-granite 

 and interlaminated mica-schist, in apparently equal proportions, has a 

 specific gravity of 2 "74. Specimens in which far more subtle inter- 

 mingling has gone on have much the same density, despite the more 

 coarse development of their crystalline constituents. The crystalline 

 associations probably arise at an early stage, and the growth of larger 

 crystals is a process of rearrangement of the groups already formed. 



The contact-zone occasionally shows a yellow-brown composite rock, 

 in which brown mica is abundant, but in which granitic characters on 

 the whole predominate. This would be styled by French authors a 

 Leptynolite.^ The biotite, which is almost uniaxial, and which dis- 

 plays grey-brown to rich yellow-brown face-colours, does not occiu* in 

 the granite itself, nor is it the same variety as that in the adjacent 

 mica-schist. Muscovite is also present in the leptynolite, and separated 

 out a little before the biotite ; the latter certainly does not represent 

 in this case mere patches of residual material derived from the mica- 

 schist. The complete graduation of this rock into the granite, and 

 also into various types of interlaminated composite gneiss, makes it 

 clear that it also is essentially a composite rock, in which absorption of 

 the schist and recrystallisation have occurred. A " leptynolite," or 

 more precisely, a fine-grained "granitite" with oligoclase and two 

 micas, and a specific gravity of 2*70, has resulted from an interming- 

 ling that must have amounted in this case to interfusion (PI. rv., fig. 1). 

 This zone of leptynolite, as observed by myself east of Cashelgolan 

 Hill, is not more than 10 cm. thick ; but I see no reason to doiibt that 

 similar effects may be produced elsewhere on a far more important 

 scale. 



Those who, with Mr. T. D. Adams, ^ have urged that such leptyno- 

 lites arise from progressive metamorphism of the constituents of a shale, 



1 Salomon, " Geologische Studien am Monte Aviolo, " Zeitschr. d. deutsch. 

 geol. Gesell., Bd. xlii. (1890), p. 471, describes the formation of a brown biotite 

 at the expense of chlorite, in phyllites invaded by the tonalite of Monte Adamello. 



- Compare Lacroix, op. cit. (1898), p. 8, and his pi. I, figs. 1-5,'aud W. Salomon, 

 " Essai de nomenclature des roches metamorphiques de contact," CongrSs geol. 

 internat., Comptes rendus, v'm'^ session (1901), p. 343. 



^ " The excursion to the Pyrenees in connection -with the 8th International 

 Geological Congress, " Journal of Geology, vol. ix. (1901), p. 44. 



