266 PEOCEEDIXGS OF THE GEOLOGICAL SOCIETY. [Feb. 5, 



thrown oif in contact with the granite ; but generally, and, indeed, 

 almost universally, there is no difference in the dip of the strata either 

 under or above the porphyritic ridges ; and not unfrequently the two 

 rocks may be seen in juxtaposition, the slates dipping underneath 

 the porphyritic masses, and again lying at the back of those masses 

 and resting upon them. In the fourth place, in the quarries of this 

 porphyry, at least in one quarry, abundant fragmeuts of the slates are 

 found imbedded, having evidently been taken up by, and involved 

 in, the flow of the porphjry when it was rising through or being 

 squeezed through the passages of the falling strata. In the fifth place, 

 these fragments are of precisely the same mineral character as the 

 parent rocks from which they have come ; they exhibit the sharp 

 angles of original fracture as fresh as if they had been broken off 

 yesterday, and have evidently never been themselves altered, nor 

 have they effected any alteration in the paste into which they fell. 

 I have already remarked on the immense importance which attaches 

 to the question of the mineral condition of the slates of the West 

 Highlands at the time when subterranean movements folded them 

 and contorted them, and drove them into waves like water driven by 

 the wind. If that mineral condition was at all like that in which 

 they now exist ; if they had already attained a hard and crystalline 

 structure, then their liability to fracture along the Hues of extreme 

 tension must have been very great, llechanical laws render it cer- 

 tain that such fractures must have accompanied the strain and pres- 

 sure to which they were subjected. The evidence, therefore, which 

 shows that the slates were just what they now are in texture at a 

 time when the subjacent porphyries were in a soft and viscid condi- 

 tion, and the fiu'ther evidence which shows that (in strict conformity 

 with the results we should expect from the respective conditions of 

 the two mineral masses) the porphyries were splashed up along the 

 lines of deposit as the slates fell inwards — this evidence, I say, throws 

 a flood of light on the whole problem we are now considering. It 

 shows that the mineral condition of the pori^hyries was precisely such 

 as would facilitate the transmission of earthquake waves ; and it 

 shows not less clearly that the mineral condition of the slates was 

 such as necessitated fractures and dislocations when such waves were 

 propagated underneath them. 



But whatever may be the import and interpretation of such facts 

 as these, certain it is that ITr. Geikie takes not the slightest notice 

 of their existence. The veiy illustrations he selects in support of 

 his own theory show that he has paid no attention to the whole class 

 of facts connected with the relative position of the mineral masses. 

 Let me take as a conspicuous instance the Pass of the Awe. Mr. 

 Geikie mentions it as an example of his favourite theory that trans- 

 verse valleys have been cut backwards by two streams originally 

 running in opposite directions, until by the destruction of the divid- 

 ing ridge one long continuous glen has been the last result. The 

 very words in which he describes this Pass give an altogether erro- 

 neous idea of its character as a feature in physical geography. " The 

 present outflow of the lake through the deep narrow gorge of the 



