RHYTHMS IN SEDIMENTATION 801 



tures in nature far more complex than the simple diagram. Neverthe- 

 less, the average or aggregate effect is well represented. 



If the sand swept b}' wind from the source in the stony desert or 

 hamada, or brought by streams from mountains, is accumulating uni- 

 formly over a distance of a hundred miles, then over the first mile only 

 1 per cent will remain and 99 per cent will be carried beyond. As 

 equable deposition can not be postulated, there will be many times when 

 scour rather than deposition takes place in the first mile. Tlie exceptional 

 winds of a single year may scour down to the layers deposited in a previous 

 century and a few following years of slack winds may more than fill in all 

 of the scour. Thus it would be a common feature for C of the diagram 

 to come to lie below B or A, or even below far lower beds. If C should 

 cut dovni below B, it would bring lamina C21 into contact with A 3, 4, 

 or 5. 



It is seen that the average length of the time break, or diastem, 

 between beds depends on the ratio of the volume of material of that kind 

 which is carried beyond to the volume which is deposited in the locality 

 of the bed, but on account of the lack of uniformity which exists in 

 Nature the actual value of an individual break might range from nothing 

 to many times the average. The conclusion of far-reaching importance 

 is that coarse detritus and rapid accumulation of individual beds is no 

 criterion of rapid accumulation of the formation as a whole. This needs 

 to be emphasized because the natural tendency, without analysis of the 

 problem, is for geologists to assume the contrary to be true. 



Looking at the farther side of the region of deposition, it is seen that 

 during the maxima of transporting power there is only deposition and 

 no removal of material; but at times when the wind is less intense, there 

 is no transportation of material of this grade of coarseness to-^uch a 

 region. Thus it may be taken as a general proposition that the coarser 

 the waste and the more* rapid the accumulation of individual beds, the 

 longer, relatively, are the times of non-deposition. Furthermore, it is 

 seen that on the side of the formation toward the source the break, or 

 diastem, is due to an accentuation of the carrying forces. On the side 

 away from the source the lack of deposit is due to the alternating qui- 

 escence in transportation. The diastems in one locality are represented 

 by beds in another locality in the same formation. Each section may 

 show a high degree of discontinuity, and yet i(: various sectious could be 

 pieced together a more or less complete record might be attained. 



The rate of accumulation of cougloineratc and sandstone formations, 

 as well as that of the individual sand beds, may l)e on the average more 

 rapid than tliat of argillaceous formations, and these in turn possess a 



