162 STRUCTURE OF LACUSTRINE STRATA. 



materials are poured from a height, they usually arrange themselves in 

 a conical heap, whose sides make nearly this angle with the horizon. 

 On the slopes of mountains liable to avalanches or rapid waste, the 

 loose debris is usually found in a plane declining at about this angle. 

 When streams falling over a ledge, transport with their waters a quantity 

 of earthy matter, the conical heap so thrown down is very much more 

 obtuse than when the materials fall dry, because their weight in water 

 is less, so that the fall of the particles is not so direct, and the larger 

 the proportion of water that comes down, and the more forcibly it 

 descends, the flatter is the slope of the cone. This will easily be 

 understood upon the principle that by partial suspension in water 

 each particle is influenced by the tendency of that fluid to become 

 level. 



It is easy to understand from this that the form in which coarse 

 sediment will be deposited by rivers entering a lake, must be in a very 

 obtuse cone, radiating round the point of entrance. As the heap of 

 sediment is advanced into the lake by continual additions, its outline 

 remains circular, with a larger radius, and its section will be nearly 

 level toward the land, but sloping more and more rapidly toward the 

 interior of the lake. Were the particles to be arranged in obedience 

 to the double forces of horizontal movement with the river, and of 

 perpendicular descent from gravitation, the curve of the edge would 

 be parabolic, arid the surface left upon the sediment toward the land, 

 nearly level. 



But the earthy matter being unable to support itself at more than 

 a certain angle of elevation, the lower part of the curve will become 

 less steep, and be reduced to a straight line. Observations on the 

 Swiss lakes assign to the sediment left therein an outline of this 

 kind. 



It is obvious that in these cases the sloping layers nearest the 

 entrance of the stream are of older date than those farther advanced 

 into the lake. It is an interesting subject of inquiry to learn whether, 

 as is most probable, the particles of the sediment which differ in bulk 

 and specific gravity, are arranged according to those qualities so as 

 to constitute horizontal strata, of finer and coarser matter, &c. ; and 

 whether, this being the case, the sloping lines of deposition, &c., are 

 visible or obliterated in the section. In this manner the upper ends 

 of lakes are filled almost to the surface with the deposits from the 

 rivers ; and the dams of the lower ends of the lakes being worn 

 away by the incessant action of the stream, these deposits become 

 visible above the water, and constitute those smoothly declining, often 

 moist surfaces, which usually confine within their indefinite border 

 the shallow and weedy waters, destined in their turn to retreat from 

 the desiccated land. While this process proceeds near the shore with 

 the coarser particles, it is obvious that the finer sediment will be car- 

 ried farther into the lake, and be spread more widely over its general 

 bed. 



These remarks apply only to deep lakes, whose waters rest tran 

 quilly on their beds, and are only agitated at the surface. In shallow 



