CONYEESIOX OP CEYSTALLIXE SCHISTS INTO IGNEOUS EOCES. 517 



36. On the Alleged Costeesion of Ceystallixe Schists into 

 Igxeoes Pocxs in County Galway. By C. Callaway, Esq., 

 D.Sc, P.G.S. (Pead April 6, 1887.) 



Introduction. 



1. General Distribution of the Igneous Eocks. 



2. The Relations of the Igneous Eocks to each other and to the Schists. 



Knockseefin, Ground south-east of Glendalough, Lettershinna, 

 Ground south of Glendalough. 



3. The Foliation of the Igneous Eocks. 



The Granite. The Diorite. Veins of an Acidic Eock. 



4. Age of the Igneous Eocks. 



5. The Galway Gneiss. 



6. Summary. 



Ixteoductiox. 



The theory of the metamorphism of aqueous deposits into granite and 

 other igneous rocks has been maintained by most Irish, geologists, 

 and is set forth with much detail in the elaborate memoirs of the 

 Irish Geological Survey * on the district between Galway and 

 Westport. It therefore seemed to me, after some preliminary work 

 in Donegal f, that the Connemara region would probably afford 

 rich material for the determination of the question. In this 

 hope I was not disappointed ; but while working at the relations 

 between the igneous and the metamorphic rocks, another problem, 

 the origin of the schists themselves, began to emerge. The result 

 of my inquiries was a singular reversal of the theory I was exam- 

 ining. I found, not that the igneous rocks had been formed out of 

 schists, but that some, at least, of the schists had been formed out of 

 igneous rocks. The Galway region has, indeed, been most fruitful 

 in suggestion, and has supplied me with a clue to the origin of some 

 of the less complex gneisses. 



I have to acknowledge my obligations to Prof. Bonney, P.P.S., 

 who has been kind enough to look through my microscopic slides, 

 and to give me his opinion on critical points. 



In discussing the origin of the igneous rocks, it will be desirable 

 to state briefly the evidence upon which the theory of the Irish 

 Survey has been based. I will give the chief points, as far as 

 possible, in the words of the Survey lEemoirsj:. 



Of the granite there are three types — Intrusive, Porphyritic, and 

 Poliated. The porphyritic passes into the porphyritic-foliated, and 

 the latter sometimes gradually loses its porphyritic character, and 

 seems to pass into gneiss and schists. It is inferred as probable that 

 the foliation of the granite points to its original stratification and that 

 even the porphyritic granite was originally part of a sedimentary 

 series ; but being nearest to the " seat of metamorphic action," " all 



* To accompany sheets 93, 94, 95, 104, 105, 113, 114. 

 t Quart. Journ. Geol. Soc. toI. xli. p. 221. 

 | Memoirs 105 and 114, pp. 7-16 et alibi. 



