RHYTHMS IN SEDIMENTATION 797 



tion of such effects for three combined rh3^thnis^ such as have been dis- 

 cussed, of successively smaller amplitude and shorter period. 



On the left is the columnar section of the strata which result from these 

 combined oscillatioiis. The light lines of separation are the bedding 

 planes, or diastems, and in the curve as here drawn represent lost time 

 intervals about equal to that recorded by the beds. The heavy horizontal 

 lines are disconformities due to cessation of sedimentation and down- 

 scouring during the recurrent longer stages of sinking baselevel. On the 

 curve as here drawn they represent considerably longer times than the 

 intervening formations. Lost intervals of more than half the time neces- 

 sarily occur for harmonic oscillations, if the trough of a secondary rhythm 

 is lower than its preceding crest; in other words, if the slope of the limb 

 of the secondary rhythm in descending phase is steeper than the slope of 

 the primary rhythm in opposite phase. On the other hand, if the minor 

 rhythms have very long period and low amplitude they will not interrupt 

 the cojitinuity of sedimentation on the ascending limb of the next higher 

 rhythm, but will cause only variations of rate. There will be bedding, but 

 no diastems. 



The evidence of recent climatic and crustal movements indicates that 

 the changes are relatively rapid, separated by intervals of quiet. The com- 

 l)ined curve would diif er from the simple harmonic form by showing longer 

 flats and sharper vertical movements. Where sedimentation is closely 

 controlled by baselevel it would mean, a necessary discontinuity, or diastem, 

 involved in each rhythm. The assumed harmonic motion, by giving 

 gradual slopes to the curves, is apparently more favorable for giving 

 continuity of record than are the more abrupt changes of nature. In 

 figure 5 the columnar section looks not unlike those found in nature ; 5^et 

 only one-sixth of the time is recorded in it, as shown by the bh^ick bars 

 above the curve. The actual material existence of the strata is so much 

 more impressive than the division planes between them that this diagram 

 is necessary to show how for this case the strata give a recorri for only 

 a small fraction of the time. 



The rapidly subsiding center of a geosyncline would correspond to a 

 steepening of the curve A A and a laiger proportion of the time would 

 be recorded than on its landwai'd margin. Where the diastrophic oscilla- 

 tions are large as c()m])ar(Ml to the progressive change of baselevel, as near 

 the margin of sedimentation, or in places of Init little progressive sul)- 

 mergence, the record is, on the other hand, very discontinuous. Smaller 

 oscillations are in part of a diastrophic nature and are compounded of 

 changes in the level of sea and land. Many small oscillations and some 

 of even larger magnitude ai-c, however, dependent upon climatic change. 



