14 Rev. J. F. Blake — Form of Sedimentary Deposits. 



supply. This original theoretical form will, of coui'se, be mucli 

 modified in nature by various other causes, but it will not, on the 

 average, be obliterated. 



Fig. 1. — Section of Deposit of large Detritus at a Eiver's Mouth. 



A-£ = sea-level ; C-D, bottom of the current ; H-F, shelving bottom of the sea ; 



a-b-F, section of the deposit. 



Now consider the case of the material in suspension, which is the 

 crux of the whole question. So long as the current is strong enough 

 to push along the heavier particles, it is a fortiori strong enough to 

 hold up the lighter in suspension. None, therefore, of the latter 

 (except the diminished stones), will begin to sink till after the 

 former has settled, and the whole of the finer deposit will lie 

 seaward of the coarser. At a later time the coarser deposit may 

 grow over the earlier part of the finer, but the two parts that are 

 brought by the same body of water will always be initially separate. 

 As the stream may possibly contain in suspension particles of various 

 rates of sinking, we must consider these sepai-ately, and temporarily 

 assume for working out the result that the rate of sinking of 

 the part considered is constant in water of given velocity. The 

 velocity of the stream on reaching the sea will be at last reduced to 

 the stage at which sinking commences, and thereafter it will be 

 continually more and more reduced till it becomes practically zero. 

 We have seen that the result of this diminution of velocity is 

 twofold. It causes the sediment to fall more vertically and against 

 a smaller resistance. 



Now the sediment may be considered as arriving in a series 

 of vertical sheets transverse to the stream, and each sheet, as the 

 particles in it sink down, will be spread out horizontally on the 

 bottom. Let us take a section of such a sheet, a-e, Fig. 2, and 

 trace the course of the particles in it across a series of equal 

 divisions, in each of which the horizontal velocity is assumed to be 

 constant, so that the paths of the particles in each are all parallel ; 

 but in successive ones these velocities are decreased, so that the paths, 

 though still parallel amongst themselves, will be more inclined than 

 in the preceding division. Selecting particles which thus arrive at 

 points equidistant from each other, as b", c", d", so that the line 

 Ij"c" = c"d", since d'd" is more inclined than c'c", it follows that 

 c"d' is greater than b"c', and therefore that dc is greater than be. 

 In other words, if the sediment is uniformly distributed in ae 

 a larger proportion of it will be deposited between d" and c" than 

 between the pi-eceding equally distant points c" and b", or the deposit 



