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Proceedings of the Royal Irish Acacle^ny. 



ref erring to it, the metamorpliic series lying to the north and south of 

 the Silurian basin must be described, so that the elements of the 

 geology of the entire region may appear in duly ascending order. 



The complexity of the stratigraphy of the Silurian tract may well 

 account for incomplete appreciation of its geological structure at the 

 outset ; and a similar reflection d fortiori applies in the much more 

 intricate case of the dislocated, over-folded, and highly altered rocks — 

 chiefly crystalline schists, quartzites, and igneous masses, most of them 

 deformed — which form the crust in JSTorth Mayo and Connemara. 

 Since the publication of the Government maps and memoirs of these 

 regions, a few papers referring to their geology have appeared, as well 

 as references in the yearly Summary of Progress, according as increase 

 of light seemed to justify different views of the structure. It has 

 been generally recognized by the Survey staff that deficient light 

 upon structural geology and metamorphism had tended to render the 

 original mapping considerably obscure. Thus, since the year 1881, 

 the application of Professor Heim's Alpine observations to !N'orth- 

 West Scotland, by Professor Lapworth and the officers of the Scottish 

 Survey, under the direction of Sir A. Geikie, resulting in the discovery 

 of transformations effected in jS'orth-West Scotland, by movements 

 along a series of successively out-cropping thrust-planes, had thrown 

 a flood of light upon the geology of the Irish metamorphic regions. 

 This was availed of in the interpretation of West Donegal, which was 

 mapped in the years 1885-90, and in the revision of the correspond- 

 ing metamorphic regions of Mayo and Galway. 



Another factor which has especially contributed to differences 

 between recent and older views of our rocks is the effect of dyna- 

 mical metamorphism ; this was not at all realized when Mayo and 

 Galway were originally surveyed : I refer to the mylonization or 

 shearing of igneous and sedimentary rocks alike, attended with the 

 formation of mica at the expense of the felspars, and the consequent 

 production of gneiss and mica-schist from granite masses. 



Until the close of the seventies in last century some of the granites 

 were regarded, in Ireland at least, as the extreme limit or climax of 

 metamorphism of sedimentary strata ;^ and when dynamical metamor- 



^ That gneisses are, in some instances, due to the metamorphism of sedimen- 

 tary strata is now claimed. Yan Hise writes (Monograph on Metamorphism, 

 p. 783) : — " I propose to confine the term ' gneiss ' strictly to its structural sense, 

 including all finely-handed crystalline rocks, whether of igneous or aqueous origin.'' ^ 

 And as regards granite, he has the term gneiss-granite, as well as granite-gneiss ; 



