THE JOURNAL OF GEOLOGY. 



versa, but such suggestions need to be qualified by considering 

 the probable fetch of the waves, the corresponding initial strength 

 of the undertow and the declivity of the seaward slope. 



A thin stratum of coarse cross stratified sands may represent 

 a transgression by a beach-building sea over a subsiding land. 

 A thicker stratum may have been formed by deposits from under- 

 tow behind a stationary or advancing beach line, and if such a 

 deposit shows cross-stratification throughout, it was washed by 

 conflicting currents, probably tidal, during its accumulation. 



The deposition of beach-washed sands is consistent with con- 

 stant or subsiding level of the land in relation to the sea. It 

 does not appear that it is likely to occur during uplift from the 

 sea except in the comparatively rare case of the rapid ele- 

 vation of a bold coast range with preponderance of rock-break- 

 ing over rock-decay. 



The occurrence of a stratum of sandstone is not evidence that 

 during its formation the land furnished no other detritus. If the 

 sands be of mixed mineralogical composition, bold declivities on 

 land and prevalence of rock-breaking are indicated ; but if the 

 sands be chiefly quartzose it is more probable that the waves 

 have sorted the waste of a residual mantle. 



Quiet Water. — [b) When a current enters a body of quiet 

 fresh water, unvexed by tides or winds, as a stream enters 

 a lake, the inertia of the greater mass and the diffusion of 

 the stream in the greater volume checks the current, and it drops 

 whatever sediment it may have carried. The laws of this simple 

 case can be formulated mathematically, and Babbage has calcu- 

 lated the distance to which sediments of an assumed character 

 would be transported by a river current of assumed velocity 

 entering a salt-water body, whose bottom has an assumed slope ; 

 he neglects the difference of density between fresh and salt 

 water, and assumes an off-shore current equal to that of the river 

 at its mouth. 1 The conclusion is determined in advance, and 

 cannot be applied to the interpretation even of lake sediments, 

 since the assumed conditions of sediment and current are hypo- 

 1 Hand Book of Physical Geology, 1884. A. J. Jukes-Browne, p. 185. 



