Geological Society. 49 



base of the encroaching cone. This form the author designates 

 as the obtuse cone dipt at the base. 



Narrow valleys and plains are frequently divided by transverse 

 ledges of gravel. The formation of these is attributed to the opera- 

 tion of rivers, which it is supposed had first accumulated their de- 

 tritus in dams, and that these dams, having been successively broken 

 down after the subsidence of floods, were re-produced upon a rise 

 of the streams. 



Numerous causes are assigned which vary the depth of streams. 

 These are, rains; the melting of Alpine snows and glaciers; the 

 breaking up of ice in rivers; and the bursting of lakes. 



III. Whenever detritus is conveyed by running into standing 

 water, a separation takes place between those finer particles which 

 are held in suspension, and those which it only rolls along the 

 bottom. 



As the debris of horizontally stratified rocks forms a length- 

 ened talus at their base, so the loose and heavy materials washed 

 down the side of a mountain, and conveyed into a lake, as soon as 

 they reach its margin fall in a steep slope of the same description. 

 Layer after layer is thus deposited, the result of which is, that a 

 terrace is gradually formed, dipping under the surface of the lake 

 with a gentle slope, and then abruptly terminating in a steep de- 

 clivity. 



The author next endeavours to show, that what is commonly 

 called a Delta is more strictly speaking the Sector of a Circle. 



After describing numerous examples of forms of alluvial matter, in 

 artificial reservoirs and in lakes, the author alludes to the probable 

 existence of similar deposits upon a vast scale in the deep and still 

 waters of the ocean; and considering the English, St. George's and 

 Bristol Channels, to be of the nature of estuaries, he observes, that 

 the arc of the Sector is found encircling the south-western extremity 

 of Ireland on the one hand, and the north-western angle of France on 

 the other, and coinciding with a line along which the water deepens 

 suddenly from one to more than two hundred French fathoms. 



It is then shown that lakes are filled up, not by depositions in 

 their deep, central water, but by the gradual advance of all their 

 lateral terraces and cones. 



IV. When two streams meet, they neutralize each other's mo- 

 tion, and a deposition takes place at the point of quiescence. 



Peculiar appearances ensue, when streams meet at different levels. 

 If a lateral stream brings down a disproportionate quantity of de- 

 tritus, its bed is raised, but is abruptly terminated by the action 

 of the principal stream. Hence the valleys of mountainous re- 

 gions exhibit not only level terraces formed in lakes, but others the 

 edge of which have a steep declivity. 



Finally, the author presumes that the forms which alluvium puts 

 on in rivers, are produced also in seas, and in the ocean, by the 

 opposition and union of currents flowing either at the same or at 

 different levels. 



A short Memoir was then read, entitled " Remarks on the Ex- 

 istence of Anoplotherium and Palaeotherium in the lower Fresh- 



N.S. Vol. 9. No. 49. Jan. 1831. H water 



