RHYTHMS IN SEDIMENTATION 793 



shales do not represent continuons deposition where they are thickest, for 

 there was much clastic sediment ^carried be3^ond the axis of the geosyn- 

 cline. The surface was therefore kept near baselevel, and the minor 

 rhythms must have exercised a scouring action during a part of their 

 cycle. The limestones most conspicuously show the absence of a con- 

 tinuous record. They are shallow-water deposits; the successive beds are 

 often in great contrast, and innumerable laminae show, in longer and 

 shorter cycles, oscillation of conditions. 



But the southeastern side of this trough of sediments has been up- 

 lifted and eroded, with the result that the thickest outcrops of the several 

 I'ormations are commonly the easternmost. If the record was discon- 

 tinuous in this region of maximum thickness, it was presumably more 

 discontinuous in the portions to the southeastward which are now de- 

 stroyed. The deposits of a continental shelf, if traced outward far enough, 

 must begin to shoAv a continuous record. Those of a geosyncline, even 

 one containing a niaxinuim depth of 6 to 8 miles of sediments, may be 

 almost throughout a discontinuous record. The thickest formations, al- 

 though rapidly accumulated, are in most cases of such a nature that a 

 continuous deposition can not by any means be granted. The coarseness 

 of such very thick formations is no criterion of continuity; in fact, it 

 favors the opposite interpretation. The coarseness measures the strength 

 of the streams or waves. Where these are strong, there must, as argued 

 before, be much material carried beyond that point; and, corresponding 

 to rhythmic ebb and flood of carrying force, there must have been alter- 

 nate fill and scour. 



If this interpretation be true, it is seen how unjustified is a procedure 

 which assumes a certain rate of deposition for the thickest formations — 

 a foot per century, as Sollas has taken, and multiplies this by the thick- 

 ness to determine the time of deposition, making no allowance for the 

 longer and shorter doAvn-scouring phases. 



A careful study of the stratigraphic record has brought to light many 

 breaks in deposition Avithin series of similar beds which lie on each other 

 in parallel fashion. These breaks are recognized by noting a sudden 

 change in the fauna or flora at a certain plane, or a sudden change in 

 the nature of the sediments, especially where a coarser layer lies on one 

 of finer grain. The time interval may be great, as where the totally dis- 

 tinct fauna of a later period rests on beds containing the earlier fauna; 

 or the faunal change may be slight, as where in one bed of limestone 

 some species are the same as in the bed below, but others are replaced by 

 now species. Such a plane representing a lost interval has been named 



