GK.NKKAL CONCLUSIONS. Gl 



the same limits of velocity a lighter mediiun will not 

 move such large frag-ments as a heavier. Water currents 

 dislodge masses immensely greater than the largest 

 pebble in these samples. As a result of this restriction on 

 the work of the atmosphere, its deposits are necessarily 

 less divei'se in their mechanical composition than those 

 of water. 



Another circumstance, which increases the uniformity 

 of atmospheric sediments, is the great effectiveness of the 

 atmosphere as a sorting agent. In different media the 

 sorting power increases with the decrease of the carrying- 

 power. It is a familiar fact, that moving glacier ice can 

 effect no sorting. In the same way a highly viscous liquid 

 is a bad sorter, for its motion is slow, and the small 

 particles it carries are not brought sufficiently far ahead 

 of the larger ones. 



In a current of water the velocity is greater and the 

 different grades of fragments are farther removed from 

 each other in a horizontal direction, before all have time 

 to sink. In the much lighter air this separation is still 

 wider, owing to the higher A-elocities which obtain, and 

 .still more perfect sorting is the result. Whatever the air 

 lacks in viscosity and weight must be made up by veloc- 

 ity of its currents, if any material at all shall be trans- 

 ported. 



It might be inferred that this great sorting power of 

 the atmosphere should produce diversity rather than 

 uniformity in the deposits.*) Such is indeed the case 

 whenever the load, dropped during each transient period 

 of somewhat uniform velocity', is sufficient in amount to 



*) See letter from Prof. Dana, Journal of Geology, A'ol. Ill, p. 342. 



