48 Geological Society -. 



ordinary meeting from the first and third Fridays in each month, 

 from November to June, inclusive, to the alternate Wednesdays, 

 the Society assembled on this evening for the session. 



The reading of a paper entitled " Remarks on the Formation of 

 Alluvial Deposits," by the Rev. James Yates, M.A. F.L.S.,F.G.S., 

 was begun. 



Nov. 17. The reading of the paper on the Formation of Alluvial 

 Deposits, by the Rev. James Yates, begun at the last meeting, was 

 concluded. 



After adverting to the importance of this branch of Geology to 

 the successful study of all the more ancient sedimentary deposits, 

 and to the explanation of the methods by which bare rocks are 

 converted into productive soils, the author proposes to describe 

 some of the processes which regulate the production of alluvium, 

 and the principal forms which it assumes. 



I. He considers first those processes of disintegration, not de- 

 pendent upon the action of running water, by which materials 

 are supplied for the formation of alluvium. These are of two 

 kinds. 



1. Earthquakes and landslips, by which large masses are detached 

 suddenly from the mountains, and fall occasionally with so great 

 an impetus as to extend across valleys. 



2. Other processes, such as frost and oxidation, which are far 

 more important in their effects. The agents of this class always di- 

 vide rocks according to their natural structure of separation, so 

 that every fragment of the debris is bounded by the plane of its 

 cleavage. The fragments as they fall produce two principal forms ; 

 (a) the lengthened talus, which in general covers the base of all 

 calcareous, and conglomerate or sedimentary rocks ; and () the 

 acute cone, which is discharged from the ravines of highly inclined 

 schistose rocks, having a cleavage which meets the planes of stra- 

 tification at an acute angle. 



II. The materials thus furnished are distributed by streams, 

 which round off their angles by continual friction, so as to convert 

 them into pebbles, sand, and mud. The hard and heavy fragments 

 driven along by streams, also wear down the rocks in place, the 

 latter being acted upon according to their degrees of softness and 

 their proneness to disintegration. 



When the detritus thus produced is discharged from a lateral 

 into a principal ravine, or valley, the divergence of the stream gives 

 it the form of a cone ; but as the force of running water carries 

 loose materials much further than they would fall by their own 

 weight, the form thus produced is not an acute but an obtuse cone. 

 In the Alps some of these obtuse cones attain 500 feet in height, 

 and three miles in diameter, bearing upon their surfaces forests 

 and villages. 



The quantity of solid materials descending over the apex of an 

 obtuse cone, is sometimes so great as to stop up the valley. The 

 waters of the principal stream then accumulate above the obstruc- 

 tion, and after the subsidence of the lateral stream, tear away the 



base 



