332 PEOF. J. PRESTWICH ON THE EA.ISED 



gradual, yet sufficiently rapid to prevent wave- action from removing 

 the whole of the Blown Sands, or from forming terraces, which it 

 would have done had the fall been prolonged or subject to long 

 interruptions. For the same reason no portion of the strand was 

 washed on to the land. 



The absence of marine shells in the submerged land may seem a 

 difficulty. Had the submergence been of long duration, a marine 

 fauna would necessarily have established itself; and I can only 

 account for its absence by supposing that re-elevation followed, after 

 but a short interval, on the previous subsidence; The physical 

 results of that elevation are sufficiently definite to justify our 

 assumption, and are explanatory of the conditions under which it 

 was in all probability effected. 



Mr. Hopkins ^ has shown that if a considerable area at the bottom 

 of the sea were suddenly elevated, a wave of translation accompanied 

 by a current, the velocity of which would depend principally upon 

 the depth of the sea, would diverge in all directions from the central 

 disturbance. Calculations, he says, " prove beyond all doubt that 

 paroxysmal elevations, beneath the sea, varying from 50 to 100 feet 

 in height, may produce currents of which the velocities shall vary from 

 at least 5 or 6 to 15 or 20 miles an hour, provided the depth of the 

 sea do not exceed 800 or 1000 feet." In considering the magnitude 

 of the blocks which might be moved, he found that the force exerted 

 on a surface of given magnitude increases as the square of the velocity^ 

 and that it " varies as tJie sixth 'power of the velocity of the current^ 

 But the movements must be repeated for large blocks to travel 

 beyond short distances. 



It is evident that we have in this form of disturbance an engine 

 of enormous power ; and though our hypothesis does not deal with 

 the great changes and powerful currents contemplated by Mr. 

 Hopkins, we may infer what the results might be with even a 

 fractional proportion of such changes. Movements of this character 

 would, like Nasmyth's hammer, be capable at times when the uplift 

 was rapid of exerting enormous force ; while at other times, when the 

 uplift was slow, the action might be of the most gentle character. 

 Hopkins's calculations were made for one central area of elevation, 

 and dealt with surrounding level surfaces. In the case before us 

 the area of elevation consisted of a variable and uneven land-surface, 

 so that each hill or group of hills formed a centre for the divergent 

 currents, the velocity of which would further vary according to the 

 varying gradients and lengths of the slopes. 



It follows from these premises that the character of the deposits 

 formed under such circumstances will afford a relative measure of 

 the velocity and duration of the currents under which they were 

 accumulated. Where, for example, the sediment is fine, we may 

 conclude that the velocitj' was slow and the rise which gave origin 

 to it small. "When, on the contrary, the materials are coarse, we 

 may suppose the rise to have been more rapid and the velocity of 

 the current greater. Where, again, large blocks have been trans- 

 * Quart. Journ. Geol. Soc. vol. iv. (1848) p. 90. 



