﻿186 DE. H. C. SORBY ON THE APPLICATION OF [May I908, 



Possible CoDnexion between the Structure o£ a Rock and 

 the Depth of the Water. 



If the final velocity (/) at which a grain of sand subsides in a 

 deep current is the same as in clear still water, so that my experi- 

 mental results can be utilized for calculations, it follows that the 



time taken to subside through a given depth (d) would be j seconds. 



Also, if it were carried along by a current of V miles per hour for 



a distance of L miles the time taken would be ^. If, then, we 



considered a case in which during the time a grain subsided from 



the top to the bottom, we should have d=fx^. Since/ is known 



by experiment, the depth depends on two independent variables, 

 the value of which can be roughly estimated, so as to see whether 

 the result is in any way probable. Assuming then that the grain 

 of sand is a hundredth of an inch in diameter, having a final 

 velocity of '11 foot per second, and that the distance to which it 

 can be drifted is 10 miles, by a current of 4 miles per hour, we 

 have for the depth -11x9000 = 990 feet =165 fathoms, which 

 appears to me so unreasonably great as to indicate some flaw in the 

 argument. Possibly, in subsiding in muddy water the bigger grains 

 collect into pellets with the finer and with organic matter of little 

 density, so that the rate of subsidence is much less than -11 foot 

 per second. Supposing that it were only a tenth of that, the depth 

 calculated as above would be only 16| fathoms, which is not 

 unreasonable. On this principle we could explain how fairly- 

 coarse sand could be carried for some miles and accumulate with 

 fine-grained material which had subsided from a lower level, 

 where the velocity of the current was less. Quantitative results 

 are at present out of the question, but it seems extremely probable 

 that the difference in the structure makes it possible to distinguish 

 between deposits formed from deep water and those formed from 

 shallow. It is even possible that further study would enable us 

 to form some estimate of the actual depth. 



The chief defect in some of the foregoing conclusions is that the 

 influence of the depth of the water is so imperfectly known. This 

 probably cannot alter materially the relation between the sand 

 and the current, on the actual surface of the bottom, but might 

 considerably modify the relation between this and the mean velocity 

 and that of the upper surface, so that some of the velocities given 

 may not be strictly correct. 



YII. Drift-Bedding. 



What I have always called drift-bedding is formed when sand 

 is drifted along, if the water is of proper depth and the current 

 sufficiently strong to carry it on, until it arrives where the depth 

 is so much increased, and the current so greatly reduced, that it is 



