732 J. A. UDDEX COMPOSITION OF CLASTIC SEDIMENTS 



it accmniilates on flat lands, where the rainfall is wholly absorbed by the 



vegetation, mold, and soil, and thus is ineffective as an agent of erosion. 

 The general recognition of these modes of work is not new. We find 

 words in the common vernacular that designate them; bnt, so far as I 

 know, no investigation has previously been made to ascertain accurately 

 the variations in sorting of materials resulting from different modes of 

 natural sedimentation — ^that is, the nature of the mixing of different 

 sizes of clastic elements in different kinds of sedimentary deposits. The 

 analyses made and here presented show certain general persistent fea- 

 tures ia the distribution of the quantities of materials of different coarse- 

 ness in the different deposits produced by these different modes of sedi- 

 mentation. This distribution is characteristic for the two or three modes 

 of sedimentation and different also for the two media of sedimentation, 

 and clearly indicate the general validity of the modal classifications 

 given. It seems that the measurements made warrant the statement of 

 some general laws which govern sedimentation ia these respects. 



LAW OF THE CHIEF IX&EEDIEXT 



1. In most sediments there is a mean size of clastic elements that is present 

 in greater quantity than any other size. 



The validity of this law is already evident from the fact that there 

 must be for each locality where sedimentation takes place only one pre- 

 vailing current in the medium of deposition. It is highly improbable 

 that two currents, or two alternatively successive velocities of the same 

 currents; should be . so nicely adjusted with regard to strength, time 

 limits, and supply of burden that they would deposit equal quantities of 

 their sediments at the same point. The prevailing current deposits the 

 greater part of the sediment. 



If a current were perfectly uniform its sediment might consist of par- 

 ticles of the same size. The mass of a particle capable of being trans- 

 ported by any particular current varies as the sisith power of its velocit}', 

 the material of the particle always having the same specific gravity and 

 the same form.^ But such uniformity- in natural conditions is nowhere 

 iQ existence. A more true conception of the natural conditions of sedi- 

 mentation is that it is effected in each particular locality by an infinite 

 number of currents, differing iu direction as well as in velocity and all 



8 In this paper these two quaUties are regarded as uniform unless specially stated. 

 Quartz, limestone, and feldspar have so nearly the same specific gravity that no appre- 

 ciable differences in texture caused by differences in specific gravity are likely to appear 

 in any of the measurements here made. The particles in silt and dust are always more 

 angular than sand, and this may be a factor of some importance in sedimentary sorting, 

 but in this study it has been neglected. 



