KiLKOE— Silurian and Metamorphic Rocks, 



1^3 



phism was admitted as a factor in alterations of the rocks, a disposition 

 was apparent to swing to the opposite extreme in regarding schists 

 included in the granite as the extreme deformation of portions of the 

 igneous rock, in many more cases than the circiimstances justified, 

 rather than as included masses and tongues of altered sedimentary 

 strata, containing perhaps a proportion of infiltered granitic material 

 from the surrounding magmas.^ 



To understand how these changes of view affected the reading of 

 our intricate and puzzling metamorphic areas, and ultimately led 

 to settled and, as I venture to maintain, satisfactory conclusions, 

 it is necessary to refer to the history of the opinions and contro- 

 versies which have obtained since the publication of the Government 

 maps and memoirs. 



At Erris Head, the northern extremity of the Mullet peninsula in 

 Mayo, and near the town of Belmullet, occur coarse gneisses, which 

 long had been supposed to be of Archaean Age. In the Survey Memoir 

 of this region (Sheets 39, 40, 51, 52, 62), Professor E. Hull mentioned 

 the "possible presence at Belmullet of gneiss belonging to the 

 Laurentian formation" (see preface) ; and on p. 14 of the same he 

 compared the rocks west of Belmullet with those around the village 

 of Rhiconich, and along the shores of Loch Laxford in North- West 

 Scotland. Dr. Hull also described the gneiss somewhat in detail in 

 a paper, " On the Laurentian Rocks of Donegal and other parts of 

 Ireland " ; and mentioned its occurrence in Slieve Gamph and the 

 Ox Mountains, and in South-West Galway. 



WoEK OF Revision. 



In the year 1890 the Geological Survey entered upon a revision of 

 areas already mapped ; and the Dir-ector-General, accompanied by 

 Messrs. Peach and McHenry and the late Dr. Hyland, made traverses 

 in the west, with a view to deciding as to the age of the supposed 

 Archaean rocks in the regions of Belmullet, Achill, and Galway. The 

 traverses resulted in the confirmation, at the time, of Professor Hull's 

 views as regards the similarity of the Galway rocks to the Scottish 

 Archaean — both lithologically and in their present surface features; and 



and uses the descnptive term mica-quartz-felspar-schist-granite for Aurola granite, 

 which implies his recognition of its derivation fi oni aqueous rocks. 



^ This change of view amongst some geologists is referred to by Prof. G. A. J. 

 Cole, M.R.I. A., F.G.S., Director of the Survey, in his paper, " On a Hillside in 

 Donegal," Science Progress Xo. 2, October, 1906, pp. 16, 17. 



