Distribution of Alluvium on Coasts. 37 



pecially in straits and shallows. In this view, the English 

 Channel itself is analogous to an estuary ; or rather, it may be 

 considered as one branch of a still greater estuary, which also 

 includes the Irish Sea, the Bristol Channel, and St George's 

 Channel. From the Straits of Dover, it deepens gradually 

 from 180 to rather more than 500 feet. Immediately beyond 

 this even slope, the ground sinks to more than twice its former 

 depth. Here, then, we apparently come to the boundary, where 

 the currents of various kinds cease to sweep the bottom of the 

 ocean, and ail the detritus consequently falls in a steep declivity. 

 The line which marks this sudden fall is an arc, including the 

 south-western extremity of Ireland on the one hand, and the 

 north-western angle of France on the other. In going south- 

 ward, it preserves a very exact parallelism to the line of the 

 French coast, and seems to be analogous to the shelf which is 

 found beneath the margin of a fresh-water lake *. 



With regard to the stratification of these deposites, I can only 

 form a conjecture grounded upon the process of their formation. 

 It seems that the successive masses of detritus, varying, accord- 

 ing to the quantity and force of the stream, from the finest sand 

 to the coarsest gravel, and falling over the declivity c d. Fig. 27, 

 must produce a stratification always parallel to that declivity. 

 In those Swiss lakes which are supplied by the melting of ice 

 and snow in summer, it is true, that, on the subsidence of the 

 water, which takes place every winter to the extent of about six 

 feet of perpendicular depth, masses often fall from the upper 

 part of the declivity, probably in consequence of the pressure of 

 the water with which the whole mass is charjjed, and which 



-OKI' ■ _ _ _ . . 



bursty, the sides of the declivity, wherever it is weakest and most 

 pervious to water. But the masses thus thrust out of their place 

 will fall over the lower part of the cone, making the stratifica- 

 tion more uneven, but without materially altering its angle. The 

 agitation of the water by winds must also wash down portions 

 from the top of the declivity, so as to round off the angl ej^c d ; 

 but these portions, in falling to d, will be inclined as before. 

 From other causes, however, a stratification nearly horizontal 

 will take place at the bottom and at the top of the deposite. 

 Agreeably to the observations formerly offered, the largest 

 masses will fall continually to the bottom of the slope, and will 



• The French Coasting Pilot, London, 1805, 4to. Tl. 8 and 17. 



