270 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 5, 



they would tend to break and gape. In another passage our 

 author himself admits that " the coincidence of a valley with an 

 anticlinal axis may perhaps be traceable to an actual fracture 

 of the strata along this line of severe tension." But, in the third 

 place, I have to observe that although I am not personally ac- 

 quainted with the Loch-Taj district in minute detail, I do know 

 enough to be sure that Mr. Geikie's facts are at least exceedingly 

 incomplete. As usual, he avoids aU allusion to the occurrence of 

 granite, or other rooks usually regarded as eruptive ; and from 

 his description and sketch no human being would suppose that 

 they make their appearance anywhere on the bank of Loch Tay. 

 Now I happen to know that the southern banks of Loch Tay con- 

 stitute a district of much disturbance, large masses of granite and 

 porphyritic rocks charged with metalliferous deposits appearing 

 through the slates, and indicating some great subsidence of the 

 strata. I do not, indeed, know how these strata lie with reference 

 to the granites, whether they underlie them, as in the case near 

 Inverary, or whether the slates are thrown up and tilted against 

 those masses as along the foot of Ben Cruachan. But these are 

 precisely the facts which Mr. Geikie ought to have supplied, because 

 they are the facts on which the whole question of. mechanical causa- 

 tion of hill and valley may depend. I attach no value whatever to 

 a theory which passes over and ignores this class of facts altogether. 



I now proceed to notice, in the last place, one rather obscure argu- 

 ment on which much stress is laid in support of the theory that all 

 our hills and valleys are due to erosion. Mr. Geikie says that there 

 is a wonderful symmetry in the general structure of all river- and 

 valley- systems ; one glen branches oif from another, and all con- 

 duct their waters to the common goal, which is the sea : and then, 

 in looking on the hiUs from some distant point, or from one of their 

 own higher summits, he sees a wonderful and mysterious average 

 in their height. This symmetry of structure in the glens can only be 

 due, it is said, to the trickling of water, just as we see it in wet sand 

 which the tide has left ; and so in like manner this general average of 

 height can be nothing but the once uniform level of the great mud- 

 bank when it rose above the sea. My answer to this argument is, 

 first, that the hill- and- valley system of the Highlands is not in the 

 least like the symmetry of rills oozing out of, and cutting their ot^ti 

 way through, wet sand ; secondly, that such symmetry as does exist 

 is more easily accounted for by a totally different explanation. 



Pirst, then, the symmetry, whatever it be, is not of the kind 

 which would be made by water merely cutting its own shortest way 

 to the sea. The tributary streams in the Highland glens join each 

 other and the main valleys at all sorts of angles, and from every 

 point of the compass ; and their coui^se appears to be invariably de- 

 termined by heights and ridges which they are compelled to run 

 round, because they cannot run through. In the next place, such 

 symmetry as does exist is due, by the confession of Mr. Geikie 

 himself, to geological structure ; because he admits, as we have seen, 

 that the longitudinal sj^stem of valleys foUows the strike of the 



