Denuding Agencies Under the Mow of Ice and Water. 167 



The author had measured the remanies valley gravels of Coalbrook 

 Dale, which were associated with marine shells 200 feet above the 

 sea, and compared their contour with ordinary valley gravels and 

 with marine beaches, to ascertain under what probable conditions 

 the sea had risen up the Severn valley without leaving any traces 

 of cliffs or marine denudation except between Bridgnorth and Coal- 

 brook Dale. The diamond gravel-deposits in Africa have a similar 

 contour to those of Coalbrook Dale. 



The position of the Moel-Tryfaen beds was first described by 

 Trimmer in 1831. Trimmer, an excellent geologist, observed the 

 scratches on the rocks covered by the gravel with marine remains, 

 and noticed their ice-origin, but did not draw, unfortunately, the 

 natural inference that there must have been a Glacial period in 

 Wales. This great discovery or invention was left to Agassiz to 

 propose in 1837. 



The glacier-eroded lakes, much lower than Moel Tryfaen, and 

 close to it, are free from marine remains ; therefore it seems difficult 

 to suppose a depression of 1300 feet and immersion in the sea of 

 Tryfaen, and subsequent elevation, could have taken place without 

 having left any marks on the land except at one spot. 



The measured section of the Chesil Beach shows its close approxi- 

 mation to a binomial curve • and the regularity of beaches and 

 littoral zones along the Channel teach us what are the certain con- 

 sequences of land being immersed under the sea. 



Mr. Tylor produced plans and sections showing how the tide 

 actually affects the sea-bottom, and described the gorge below 50 

 fathoms in the Irish Sea. 



He treated the tide as caused by the alternate and opposite slow 

 movement of the deep and great mass of the Atlantic, giving motion 

 to the water at the coast almost simultaneously as if the whole 

 water moved as one mass over an area of thousands of square 

 miles." The velocity of the tide of one tenth of a mile per hour 

 in the deep sea, produced by the composition of forces a tide of a 

 velocity of three or four miles an hour on the coast. High and 

 low water at different ports are the direct consequences of local 

 currents in shallow water, set in motion by the greater mass of 

 deep water. There are points in the English Channel where within 

 a few miles there is a difference of six hours in high water. He 

 objected to the theory of a tidal wave travelling in one direction, 

 and moving faster in deep water than in shallow, because the tide 

 really travels quicker in shallow water, as his plans show. 



In support of this he showed the chart of the Channel, and that 

 the tide turned in the Irish Sea at all points, deep or shallow, 

 almost simultaneously and synchronously with the slow tidal move- 

 ment in the Atlantic. He found that in a large area of sea of 

 120,000 square miles, where the water averaged 67 fathoms (off the 

 Scilly Islands) the velocity of the tide was only one mile per hour ; 

 but in the shallows near the Channel Islands, where the depth was 

 on an average 12 fathoms, by the composition of forces the velocity 

 of the tide increased to 6 miles an hour. If the tide were the con- 



