bearing on theoretical Speculations. 265 



would be likewise gradual. Now the lines indicating the main 

 direction towards which the waters in their subsidence must, 

 tend, being coincident with the dip of the strata over the 

 backs of which the descent was taking place, must of course 

 have been transverse to the bearing of those strata : the ge- 

 neral currents of the so descending waters would therefore 

 naturally tend to produce transverse furrows in the strata: 

 hence would the transverse valleys originate ; while at the 

 same time the longitudinal valleys would be materially modi- 

 fied; the descending currents setting against the escarpments 

 of the strata would naturally tend to undermine them, and 

 from the direction of the inclination of their planes would act 

 to advantage, especially as we usually find the longitudinal 

 valleys extending into the softer alternating strata, such as 

 clay, and sand, and the harder rocks constituting the over- 

 hanging escarpment: hence the undermining agency of the 

 waves operating with facility on these softer materials, would 

 considerably increase the breadth of the longitudinal valleys 

 and render the escarpments steeper and more abrupt. In 

 proportion as the depression of the sea-level was gradual, 

 there may have been a long continued reiteration of tidal waves 

 sweeping over the same tracts. I happen at the present mo- 

 ment to have directly beneath my eyes a complete illustration 

 of the necessary consequences of the action of tidal waves on 

 strata gradually inclined, residing within a few yards of a 

 coast formed by such strata (of magnesian limestone) ; these 

 dip towards the sea under a very gentle angle, only about 2. 

 The ebb consequently exposes a band of them of considerable 

 breadth, more than a furlong; the whole of this band has been 

 eroded by the tidal waves into a complete and most illustra- 

 tive model, presenting on the small scale all the phenomena 

 above described, escarpments overhanging expanded longi- 

 tudinal depressions, transverse breaches, &c. &c. And I may 

 add, that the tidal action very commonly does actually pro- 

 duce what Mr. Lyell, following Mr. Scrape, fancies can result 

 only from fluvial action, namely, serpentine and meandering 

 furrows often of considerable depth and length. I need not 

 add what pleasure it would give me to see either gentleman 

 here, and convince him of the fact by ocular demonstration. 

 I have indeed been surprised how this argument could have 

 been so strongly urged by two observers, to the combined 

 acuteness and accuracy of both of whom geology is so deeply 

 indebted : for it has ever appeared manifest to me, that even 

 a diluvial current, supposed to be excavating the strata over 

 which it rushes, can continue to pursue a straight inflexible 

 line no longer than the constitution of those strata is such as 

 N.S. Vol. 9. No. 52. April 1831. 2 M to 



