Dr. Mac Culloch on the Parallel Roads of Glen Roy. 357 



must have been, on this supposition, produced by a mass of water 

 greater than that which caused these, and we can have no reason 

 for supposing that flow attended by a less velocity. That three cur- 

 rents so different in depth should have been propelled with momenta 

 capable of producing effects so equal, is in itself a supposition con- 

 siderably improbable ; but among the other more obvious difficulties 

 attending this explanation it is unnecessary to dwell on this in par- 

 ticular. 



The form, dimension and equality visible throughout the whole, 

 present additional obstacles to the supposition that a deluge was the 

 cause of these lines. Among the numerous difficulties which 

 occur in attempting to solve their formation on this view, it 

 is perhaps a trifling one, that three successive torrents should pro- 

 duce three deposits of alluvial matter, of which the sections should 

 be so generally equal and similar : there are many of a more serious 

 nature. I have shown that the deposits in the upper part of the 

 glen in which the lines are traced, are of sharp materials mixed 

 with fine clay, while at its lower part they consist of rounded mat- 

 ter mixed with sand and gravel. If the deposit which occupies the 

 glen had consisted of foreign substances introduced by the flow of 

 a torrent, it should have been formed of rounded matter through- 

 out : in any case there should have been a similarity between the 

 two. Nor is there any reason why it should not be equally found 

 on hard and rocky ground as on a soft surface, since the forms and 

 inclinations of the hills are so often alike. 



It is an objection still more serious to this hypothesis, that the 

 thickness and disposition of the alluvia should not be affected by 

 the angular direction of the valley, and by the deviations from a 

 direct line to which it is subject. If we conceive a current flowing 

 through a channel of a bent or zigzag form, it is obvious that its 



Vol. iv. 2 z 



