Eev. J. F. Blake — Form of Sedimentary Deposits. IS 



material in relation to those forces. The forces are three, viz., the 

 horizontal currents, the vertical force of gravity, and the resistance 

 of the water to the sinking through it of the material. Of the 

 horizontal currents the most important is that produced by a river 

 running out to sea in a direction perpendicular to the shore. TJiis 

 alone, except the underdrift from breakers, brings new materials 

 to form an original deposit in the sea. Outward flowing tides, 

 however, act ultimately in a similar manner, and may be considered 

 to belong to the same type, the other type being the currents 

 pai-allel to the shore, which only shift material already brought. 

 The vertical force of gravity produces a downward velocity which 

 at every instant is combined with the horizontal velocity to make 

 the path of any particle of material a downward sloping one, and 

 the momentary paths combine into a curved one, which becomes 

 vertical when either the particle sinks below the bottom of the 

 current or the current itself is destroyed. The former limit will 

 usually be the first to be reached. Tlae resistance of the water to 

 the sinking of the material depends (1) on the coeflScient of viscosity 

 of the water, and (2) on tlie area, projected on a horizontal plane, of 

 the sinking particle ; but for our present purpose we need only note 

 that it is known to be increased by a horizontal movement of 

 the water, and therefore is continually being diminished as the water 

 comes gradually to rest. 



In relation to these forces only two kinds of material need be 

 distinguished, namely, that of which the particles are too light, small, 

 or flat to sink in water having the velocity of the given current, and 

 which are carried in suspension so long as that velocity is main- 

 tained ; and that which consists of stones or grains which are 

 only pushed along the bottom. There may be some small portion 

 of the material on the vei'ge between these two classes, which will 

 pass from the first to the second on a slight decrease of velocity, 

 but as a rule the classes are distinct. The rubbing together of two 

 stones, which is the origin of all detritus, results in the formation of 

 very fine powder and the diminution of the size of the stones ; and it 

 is only when the stones are diminished to a floating size that they 

 can form part of the suspended matter. The resulting form of 

 the deposit in these two classes, though similar, will arise from 

 different causes. 



In the case of material pushed along the bottom, the result is Yery 

 simple, and is scarcely a matter of dispute. None of it will be 

 carried further than the spot where the water becomes deeper than 

 the current. When there is no bottom to support the material it 

 will fall to rest at once, and fill up the space between the bottom of 

 the current and the bottom of the sea, as in Fig. 1. 



The deposit grows by additions along a-h, and not along a-F, just 

 in the same manner as a railway embankment grows, by tipping 

 material over the end ; it will be truncated at the end according to 

 the angle of rest of the material in sea-water, and if the sea-bottom 

 slopes gradually down it will be thickest at the point of its lower 

 termination, that is, at the point which is farthest from the source of 



