364 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



is easy of explanation. Sheets of lava when once poured out are 

 disconnected from the central source from which they came, and if 

 so poured out on the bed of the sea, where strata are in the course of 

 formation, and where similar strata continue to be formed after this 

 event, there is no difficulty in seeing that they may preserve through 

 all time and allchanges, their original conformability with the rocks 

 with which they are thus associated. 



But can we reason in the same way in respect to masses of granite ? 

 Supposing them — which it is very difficult to do — to have been 

 originally poured out upon the bed of the sea in which the slates were 

 forming, what forces can have acted so equally upon thin crusts of 

 sedimentary rock, and upon the deep-seated, deep-rooted hills of gra- 

 nite, that they should maintain through subsequent periods of great 

 disturbance exactly the same relative position? 



But, although I think such a supposition would at once appear in 

 the highest degree improbable to any one who looks at the shapes 

 and position of these great granitic masses, with due attention to what 

 we know of the nature and origin of this kind of rock, as distinguished 

 from lavas ; there are some other circumstances which seem to me to 

 offer decisive proof that such was not the mode in which the two 

 rocks came to occupy their present relative position. 



In the first place, we have clear evidence that the strata of the 

 mica-slate loere not in the course of being deposited when the granites 

 rose ; but that they had already been consolidated into hard and 

 sharp fractured rocks, from the fact that fragments of these strata 

 are found imbedded in the midst of the granite — caught up and in- 

 volved in the melted mass. At a quarry of granite in the immediate 

 vicinity of the town of Inverary, this is of very frequent occurrence ;, 

 the fragments of slate being of various sizes ; the granite fitting 

 closely in to every angle of the broken edges. 



Another decisive evidence of the posteriority of the granite to the 

 complete consolidation of the mica-slates is supplied by a circum- 

 stance, of which, however, I have only seen one example ; viz. a vein 

 or string of granite running for a few feet across the bedding of the 

 slates, at a point where the overlying strata are in close contact with 

 the back of a granitic hill. 



I take it, therefore, to be sufficiently proved, that the granite of this 

 district is posterior to the mica-slates ; and not only posterior to their 

 deposition, but so immensely posterior, that the slates had already 

 assumed very much the same mineral character which they have at 

 the present day*. The question then comes to be, how such masses 

 of intrusive rock can have effected that intrusion with so little disturb- 

 ance to the conformability of the strata through which they rose ? 



One supposition is, I apprehend, at once excluded by the facts, 

 viz. that the granites had burst up through the strata when they 

 were still in a comparatively horizontal position, because in that case 

 the intrusive force would have been exerted more or less nearly at 



* At certain points the metamorpliic sti-ata seem to have been still further 

 metamorphosed by contact with the granite.- 



