DISCUSSION OF DATA 733 



cantributing more or less to the deposit. Where the range of variation 

 is small the mechanical composition of the resulting sediment will be 

 somewhat uniform, and where this range is considerable a deposit will 

 be formed that is comparatively more heterogeneous. There will be also 

 differences in sorting by the same system of currents or two identical 

 systems of currents if such should exist. This is due to the fact that 

 sorting is a progressive process, and that the material supplied to the 

 currents is of variable composition. 



LAW OF DECREASING ADMIXTURES 



2. If we separate the mechanic elements in a sediment into a series of groups 

 (grades) containing clastic elements, having diameters Ijearing a constant ratio 

 to the diameters in their next group, we find that the mass in each group tends 

 to have a fixed ratio to the mass of the other groups. Excepting some special 

 conditions, this ratio is such that the mass of any group is smaller the more its 

 clastic elements differ in size from those of the mean size. 



This ratio may be called the index of sorting, because it is the expres- 

 sion of the degree of perfection to which the sorting out of elements 

 carried and deposited by other than the prevailing currents has been 

 effected. 



The prevailing current is, as stated, really only a conception. The pre- 

 vailing system of currents would be a more exact phrase. By the mo- 

 mentary reaction between each clastic unit and any constant current, the 

 latter will be resolved, next the unit carried into an infinite number of 

 somewhat slackened currents, and the capacity for transportation by 

 these slackened currents may be greatly reduced. Among boulders rolled 

 by coursing streams we almost always find sand and even silt. It is on 

 account of this interaction between any current and its own load or de- 

 posits that no sorting can ever be perfect, even by what may appear to be 

 a constant current. 



The index of sorting is determined by two factors : by the ratio main- 

 tained by the specific gravities of the sediment and of the transporting 

 medium and by the Jiature of the current and duration of its work. 



The former of these factors is a physical constant; the latter is an ex- 

 ceedingly variable factor. Owing to the former factor, there is a general 

 difference of texture in sediments of the atmosphere and sediments of 

 water. It is evident that if the ratio of the specific gravities of water 

 and quartz were 1:1, the largest boulders would be carried as far as the 

 finest silt. Water-logged fragments of wood, such as sawdust, twigs, 

 and logs, are left together on a beacli. If sand were only slightly heavier 

 than water, its sorting would be slow and less perfect. On the other 



