

DEPOSITION SIMULATING UPHEAVAL. 331 



Evidences that Strata were not Deposited in Inclined Layers. 



General experience assures us of the fact, that agitated water deposits 

 sediment slowly in the form of strata whose upper surfaces continually 

 tend to become horizontal. This is seen in inundations from a river, 

 in shallow and ruffled lakes, and within the low-water margin of the 

 sea. The form of the bottom influences the horizontally of the 

 upper surfaces of the deposits in such a way that where the bottom 

 is like a pit, the stratified masses above are hollow on the faces ; but 

 these effects of the original inequality are rapidly obliterated by suc- 

 cessive coats of sediment, all becoming more and more nearly hori- 

 zontal. In perfectly tranquil water, through which any fine sediment 

 is equally diffused, the depth to which this will cover any part of the 

 bed depends on the depth of the supernatant water, and on the angle 

 of rest in water of that kind of sediment. The angle of rest in air for 

 earthy substances is about 45. If a river bring sediment into agitated 

 water, deposited strata tend to become horizontal, but with a constant 

 dependence upon the point where the river enters, such that, the 

 quantity of sediment being there always accumulating, a general 

 conical slope therefrom in all directions will modify the horizon- 

 tality of the strata. Or if a river bring sediment into calm water, 

 or into water suddenly deepening, so that all its lower parts may 

 be considered as calm, the conical slopes from the point where 

 the river enters will be much more abrupt than in the former case, in 

 a certain proportion to the calmness and depth of the water. This is 

 the case in the deep lakes which receive sediment from the torrents 

 of the Alps. 



On considering these cases with reference to stratified rocks, it is 

 evident that instances coming within the class of conical deposits 

 radiating round a point can only be of very limited occurrence, not 

 likely to affect a general argument, and are, in fact, almost unknown. 

 The lacustrine deposit of the Weald of Sussex has been regarded by 

 Mr. Topley as an area of seeming upheaval, in which the thinning 

 out of the beds away from the district is a true explanation of their 

 dip. 1 It is very doubtful how far this can be recognised in the thin- 

 ning out of any marine formation, and certainly it does not apply 

 conspicuously to any class of marine deposits now in progress : at 

 the same time we must admit that, in all cases, the action of 

 the sea grows less and less sensible far from shore where the 

 water deepens, and the sediment brought by rivers and floods must 

 be formed in attenuated masses, thickest towards the shores. This 

 effect will be in exact proportion to the falling velocity of the 

 particles in water, so that pebble beaches lie in steeper slopes and 

 cover shorter breadths than sands, while fine clays will spread farther 

 into deeper water. But all these slopes in water are very gradual, 

 so that even against the rocky eastern coasts of England, the deep 

 waters have been filled up by sediments, which now assume a gently 

 declining surface under the water, and often only a moderate slopo 

 above it. 



1 Topley, Q. J. G. S., vol. xxx. p. 186. 



