410 TRANSACTIONS OP SECTION C. 



in place at the top of the granite mass and in time at the close of its advance. 

 The large size of the constituent crystals of the granite indicate that the sur- 

 rounding rock was still maintained at a high temperature. 



The features induced in the quartzite resemble those that are due in other 

 cases to regional deformation. It may be urged that they represent some such 

 crumpling during the rise of a laccolitic dome. Though such a dome, in 

 Gilbert's view, implies stretching rather than compression, pressure from one 

 side may thrust the rising beds together and produce local overfolding. On 

 the other hand, the short section exposed in Achill may be part of a synclinal 

 down-sagging. It is difficult, however, in either case to avoid associating this 

 limited field of disturbance with the proximity of the igneous rock, particu- 

 larly when other cases can be compared with it. South of Foxford, for 

 instance, in the county of Mayo, the granite of Slieve Gamph invades a series 

 of mica-schists and quartzites. Further west, and over a wide area towards 

 Castlebar, this granite has become darkened and gneissose through assimilation 

 of the schists ; but here above Kilmore its advance was stayed, and the margin 

 is cut, as usual, by veins that filled the cracks both of the main granite and 

 the metamorphosed sediments. These sediments have become, prior to the 

 shattering, crumpled and overfolded along the contact-region, and the section 

 upon the glaciated slope resembles that of a fluidal rhyolite on a highly 

 magnified scale. 



The wonderful contortion of the composite mass that forms the north end of 

 the Ox IMountains (Slieve Gamph) in the county of Leitrim gives a similar 

 impression of viscid flow. The melting of a single constituent of the invaded 

 schists, which here include amphibolites, would enable them to yield in response 

 to the pressures that were forcing the granite magma in thin sheets between them. 

 Their metamorphism is thermal, and the forces that have produced the 

 crumplings are not those of shearing acting on a solid mass, but may have 

 operated from a distance hydrostatically through the magma. 



Again, where limestones occur near granite contacts amid a series of various 

 sedimentary types, they display folded structures in an altogether exceptional 

 degree. Silicates have developed along their bedding-planes, but these have 

 become contorted and rolled upon one another as metamorphism reached its 

 maximum stage. At Maam Cross and Oughterard in the county of Galway, 

 .along the margin of the great granite mass that stretches thence southward to 

 the sea, these flow-structures are conspicuous on weathered surfaces. The 

 remarkable structures, moreover, of the ' skarn ' of Fennoscandia, though other- 

 wise explained by Triistedt^", may possibly represent an extreme stage of 

 thermal alteration acting upon ordinary limestone. 



Sederholm '° refers contortion on a large scale to the latent plasticity of the 

 rocks in the deeper layers of the crust, that is, in the ' plastosphere,' where the 

 pressure is sufficient to translate this plasticity into flow. His researches on the 

 basement-levels of the crust suggest, moreover, that in the depths heating 

 becomes a very important factor in aiding this plasticity. 



The maiii object of the foregoing discussion is to point out that the Huttonian 

 cycle, in which thermal changes play so large a part, implies a serious weakening 

 of the crust as magmas advance into it from below. The extensive meta 

 morphism of the pre-Cambrian strata, which amounts to a distinctive feature, 

 must, I think, be attributed, not to special intensity of tangential pressures in 

 earlv times, but to frequency of igneous attack. Much of the crumpling of our 

 schists may result from Hutton's ' softening,' the pressure being supplied from 

 superincumbent masses, or even hydrostatically, and the flow occurring laterally, 

 or^ verticallv downwards, towards regions where destruction by absorption was 

 goincr on. "The features seen during the falling in of the walls of the lava-lake 

 of Kilauea in Hawaii afford some idea of what takes place in zones of melting 

 within the crust. 



Were the scenes familiar to travellers at Kilauea to occur on a continental 

 scale, they would be regarded as part of the regular order of nature, but would 



-'0. Tn'istedt. ov. rif., Bvll . Comm. ijeol. Fivlandc No. 19 (1907), n. 91. 

 '° ' Wpitere Mitteilungen iiber Bruchspalten,' Bull. Cnrnm. n!-ol Finlande, 

 Nn. 37 {-[m?,]. n. 6(5. 



