570 Mr. J. Beete Jukes and the Rev. Samuel Haughton on the 



with the granite, sometimes, though rarely, passing into gneiss ; and often full 

 of crystals of schorl, andalusite, and other minerals. As we recede from the 

 granite, this crystalline character of the schist disappears, and finally fades away 

 into that of the ordinary dull argillaceous slate of the rest of the country. The 

 change from crystalline schist into clay slate is sometimes spread over the space 

 of one or two miles ; sometimes takes place within one or two hundred yards. 

 The altered character is always more extensive on the S. E. margin of the 

 main mass of the granite than on the N. W. side, owing probably to the more 

 rapid descent of the granite on the latter than on the former, since it often ap- 

 pears to undulate for some distance towards the S. E. at no great depth beneath 

 the surface of the ground. The alteration along the edges of the smaller 

 bosses of granite extends for a much less distance than that near the main mass 

 — the extent to which metamorphism has reached always bearing a direct and 

 obvious relation to the mass of the granite which has produced it. 



III. Period of the Action of Disturbing and Denuding Forces. 



During the time when this intrusive and metamorphic action of the granite 

 was going on, the rocks were, doubtless, being affected by some of those forces 

 of dislocation and disturbance, which then or afterwards tilted them into their 

 present highly inclined positions, and communicated to them their present strike. 

 It would be difficult, perhaps impossible, to determine whether the cooling and 

 consolidation of the granite took place early in this period of disturbance, or 

 was deferred till towards its close ; and equally difficult to determine, perhaps, 

 what share the expansion from the heat of the granite alone, or the contraction 

 consequent on its cooling, had in causing these disturbances. 



We may, however, feel pretty certain that this cooling and consolidation were 

 complete before the action of those denuding agencies had proceeded to any 

 great length, which afterwards caused the granite to appear at the surface 

 Everything we know or believe of the formation of granite compels us to sup- 

 pose that at the period of its consolidation from a mass of molten matter into a 

 crystalline rock it was still covered by at least some thousands of feet of other 

 matter. The great covering of Cambro-Siliu-ian rock, which still reposed upon 

 the granite of the Dublin and Wicklow mountains after that granite was con- 

 solidated, must, of course, have been removed by an action of denudation of the 

 same kind as that which has exposed at the surface of the earth the once deeply 



