Outlines of Geology. 1^5 



water grit, present no mechanical or mineralogical difference. 

 We may, therefore, perhaps, be sceptical, and there is much 

 ground for scepticism, concerning the supposed accuracy of dis- 

 tinction between river and sea-shells: the common test is the 

 thickness, and an extremely delicate and thin shell is, merely as 

 such, regarded as a fresh-water deposit where it happens to fall 

 in with hypothesis ; but W€ well know, that sea-shells are by no 

 «ieans uniformly thick; nor are river-shells always thin and 

 delicate. Mr. Greenough, therefore, is justified in doubting the 

 possibility of the depositing menstruum having changed without 

 any corresponding change in the deposited matters ; and in dis- 

 crediting the probability of a sea having retired before a lake, or 

 a lake having been overwhelmed and annihilated by the inun- 

 dation of the sea, without any trace of such catastrophe being any 

 where visible, on the then and still unconsolidated and unresisting 

 materials which furnished the scene of action. But the alterna^ 

 Hon is the principal difficulty, for tlie existence of lakes at levels 

 and in situations which they now no longer occupy, is rendered 

 probable by the characters and structure of the sides of hills and 

 mountains that bound certain valleys; and more especially by 

 the very remarkable appearances, constituting what are termed 

 the 'parallel roads of the mountains of Lochaber. The very extra- 

 ordinary aspect of these ridges, is such as to arrest the attention 

 of the most incurious spectator, and we cannot wonder that the 

 solitary and poetical highlander should attribute to the ideal and 

 gigantic beings of former days a v^^ork, which scorning the mimic 

 efforts of the present race, marches over the mountain and the 

 valley, and holds an undeviating course over crags and torrents. 



Tliese ridges or roads are seen in three strong lines on each side 

 "of a long, hollow, and deep valley, at a considerable elevation, 

 and corresponding exactly with each other. Among the notions 

 of their origin some are purely imaginary, others may be hypo- 

 thetically supported. It has been supposed that they were made 

 for the pleasure of certain kings of Scotland, who resided at 

 Inverlochy Castle, and near these are the kettles of Fingal, in 

 which with equal probability the royal personages are supposed 



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