230 C. CALIAWAT ON THE GEANITIC ANB 



Possible Explanations of the G^ieissic Structure. 



There appear to be three possible causes of the foliation : — 



(1) Elow-structure. The foliated granite may be regarded as a 

 hnge dyke. As the molten rock was being slowly forced up the 

 great fissure in which it lies, its motion would be more rapid in the 

 middle than at the sides. This might give rise to a vertical fluxion- 

 structure, on which foliation was subsequently induced. 



(2) Pressure during consolidation. 



(3) Pressure after consolidation. This hypothesis may take two 

 forms : — 



(a) Cleavage with superinduced foliation. The gneissic structure 

 is nearh^ vertical and as regular as cleavage. We know that rocks 

 may be cleaved, and that cleaved rocks may become foliated. In 

 this case there would be two periods of crystallization, the first when 

 the granite consolidated, the second when there was a redeposition 

 of minerals in accordance with cleavage-planes. 



(h) Pressure-foliation. Prof. Bonney, from microscopical exami- 

 nation, informs me that the granites exhibit signs of great crushing, 

 by which the original structure has been " much modified." I have 

 already (p. 228) suggested that the foliation is probably due to 

 lateral thrust ; but I am not prepared, without a more elaborate study 

 of the ground than I was able to give, to pronounce an opinion on 

 the exact mode of operation. The evidence for the existence of the 

 required pressure will be more satisfactorily estimated when the 

 schists have been described. 



II. The Schistose Eocks. 



These may be separated into two groups : — 



(1) LougJi-Foyle Series. — Semicrystalline : consisting of quartzose 

 grits ; quartzite with distinct traces of fragmental structure ; fine- 

 grained schists, some of them black ; shaly and slaty beds ; crystal- 

 line limestone and compact dolomite. Usual dip north-westerly. 

 Well exposed in that part of the Inishowen peninsula which lies west 

 of Lough Eoyle. The following extracts from Prof. Bonney's notes 

 will be seen to be confirmatory of the above description : — 

 " schistose rock ;" " might even be a phyllite ;" "fragmental struc- 

 ture still conspicuous ; " " schistose slate ; " " fragmental structure 

 very marked." 



(2) Kilmacrenan Series. — Crystalline : consisting of true quartzites, 

 quartz-schists, mica-schists, hornblende-schists, schists with hydro- 

 magnesian silicates, and crystalline limestones. Dip south-easterly. 

 Occupying a large part of the district or barony of Kilmacrenan, 

 lying to the west of the village of that name. Prof. Bonney, in 

 reference to these rocks, writes : — " I should quite agree to the idea 

 of these, or the bulk of these, being old rocks. The majority, how- 

 ever, do not seem of the oldest type." Some of the most typical 

 varieties are described by Prof. Bonney as " mica-schist," " idem, 

 with garnets," " idem, with probably some chlorite or hornblende," 

 " hornblende-schist," of " the Lizard " type, " idem, with garnet and 



