34 



Notes and Comments. 



of the final velocities of material subsiding in water is of funda- 

 mental importance ; but the relation between size of particles 

 and velocity is complex, and perhaps may be partly explained 

 by a thin, adherent film of water. The angle of rest in the case 

 of sand-grains of varying size and quality enables us to ascer- 

 tain approximately the velocity of current necessary to keep 

 such sand drifting, and that needed to move it when at rest. 

 The comparison of this angle with that observed in sedimentary 

 rocks made of similar materials may be used to dete^^mine the 

 amount of vertical contraction of rocks since deposition, the 

 average in cases studied in Tertiary and Secondary rocks being 

 from 100 to 57. 



CURRENTS AND SAND. 



In studying the drifting of sand along the bottom by cur- 

 rents (on which the author experimented in a small stream 

 many years ago), the results are found to vary, according to 

 whether the water is depositing sand as well as drifting it, and 

 according to whether ripples are or are not being formed on the 

 bottom. The velocity of a current can be determined approxi- 

 mately in feet per second for different kinds of sand. The 

 connection between the structure of ' ripple-drift ' and time 

 was discussed ; and an equation given, from which the rate of 

 deposit in inches per minute can be deduced. The connection 

 between the structure of a deposit and depth of water is found 

 to be difficult to study quantitatively. From the occurrence 

 of ' drift-bedding ' the depth of water may probably be deter- 

 mined to within a few feet, and on this being applied to par- 

 ticular rocks some interesting results come out. including the 

 se])aration of sandstones into several different groups. 



I ' CLAY DEPOSITS. 



The deposition of fine deposits, like clay, is a most complex 

 subject, varying according to the amount of mud present in 

 the water, and according to whether the grains subside separ- 

 ately or cohere together. When no pressure is appHed, even 

 when no further contraction takes place on standing for a year, 

 the amount of water included in the deposited clay may be 

 80 per cent., and when dry, the minute empty spaces may still 

 amount to 32 per cent. This leads to the conclusion that many 

 of the older rocks must now be only 20 per cent, of their original 

 thickness. In many cases there is produced by a gentle current 



Naturalist 



