RHYTHMS IN SEDIMENTATION 799 



continuous, on both a small and large scale, some discussion should be 

 added as to what are conceived to be the stratigraphic marks of such dis- 

 continuity. 



Leaving aside the significance of angular unconformities as not needing 

 amplification, we may pass to disconformities. Tliese, as pi eviously noted, 

 are not recognized by any structural discoi dance, but are marked by change 

 of faunas and represent lost intervals of greater or lesser duration; thus 

 they may represent the passage of geologic periods, or they may exist 

 between formations in the same period. Recognition of the large num- 

 ber and importance of disconformities has grown within the past decade, 

 but in their nature, like faults, they must be far more numerous than the 

 evidence can demonstrate. Careful study of the Mesozoic and Cenozoic 

 formations of the Atlantic coastal plain has shown that unconformities 

 lie between nearly every formation. In the Patuxent folio, Maryland, 

 twelve post-Triassic-pre-Pleistocene formations are listed. Of the eleven 

 surfaces of separation between them, nine are given as unconformities. 

 But formations Avhich are not separated by unconformities, as well as 

 the members within a formation, may or may not possess continuity of 

 sedimentation through the large cycle which they represent. If a transi- 

 tion zone exists between two unlike deposits, the suggestion given is one 

 of continuous deposition. This is more probable if tlu^ transition is from 

 coarse below to fine above. On the other hand, if there is a sharp con- 

 trast, a break in continuity is suggested, and this is more probable if the 

 contrast is from fine below to coarse above. 



Passing to the smaller rhythms which cliaracterize beds rather than 

 members of formations, two beds which grade from coarse below to fine 

 above are presumably continuous, but each such pair may be sejoarated 

 from adjacent pairs by a stage of scour or non-deposition, siime eacli 

 coarse bed must lie in turn on one of fiuer texture. 1 1 Avould 1)0 unsafe, 

 however, to use it as an absolute criterion of minor discontinuity, as it is 

 quite possible for sand to be washed over mud-bods witliout necessarily 

 producing a down-scour. Illustrations of tlie nature of discontinuity may 

 be given from several types of l)eds. The first will be for dunes. 



Sandstones of terrestrial origin may be produced ))y wind. The sand 

 travels in the form of dunes. As these slowly march across a tract of 

 accumulation, a region to which more sand is brought than is taken away, 

 the basal part of eacli dune may l)e left liebind and buried under some 

 following dune. The long sAveei)ing curves of the cross-bedding are planed 

 off and the angular unconformities in cioss-l)edding indicate a lack of 

 continuity of record with the sand above, giving a diastem. According 

 to shifts in tlie intensity and directions of the wind, more sand may be 



