748 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



tion. The storage of the final excess, except for locally deep water, on 

 the lands, is on the abyssal slopes of the contmental platforms, consti- 

 tuting deposits lost to observation, since it is a region which has been 

 seldom uplifted and exposed by erosion. 



Thus, sedimentation, as well as erosion, is dependent for its rhythmic 

 j-ejuvenation on changes in 'baselevel, emergence above baselevel deter- 

 mining erosion, submergeuice below baselevel giving sedimentation, either 

 of continental or marine deposits. This control of sedimentation by 

 baselevel may be summed up in three principles : 



First, the rate of sedimentation is determined usually not by the rate 

 of supply of sediment, but liy tlie rate of the discontinuous depression of 

 the surface of deposition. 



Second, subsidence of the sedimentaiy floor is not initiated by the load 

 of sediment, for depression must precede. When a movement of sub- 

 sidence has begun, however, the weight of sediment will act in the same 

 direction, but it is a secondary cause. Sedimentation, then, is the effect 

 of depression to a greater degree than it is the icause. 



Third, the deposition of a series of beds where the material is carried 

 along the bottom by stirring of currents or oscillatious of waves is 

 ordinarily not a continuous process, even during a stage of crustal de- 

 pression, but represents an irregularly rhythmic alternation of fill and 

 scour with a balance in favor of the fill. There are minor time blanks, 

 consequently, in what appears to be a continuous succession of beds, and 

 larger and larger intervals separate the larger and larger divisions of a 

 series. 



ISTumerous breaks are now known to exist in which the beds above and 

 below lie parallel, and, except for some change in fauna or flora, give 

 little or no indication of the great lapse of time which occurred between 

 their deposition. Such breaks are known as disconforuiities. The pres- 

 ent argument enlarges on this conception, holding that the breaks of 

 smaller time interval are still more numerous and may add up to equally 

 large measures of time unrecorded by sedimentation. Such breaks have 

 generally been too brief to give a clue by means of a f aunal or floral 

 change, but must be recognized through the physical features of the 

 beds, often most readily because of a sudden change at the break in the 

 character of the rhythms expressed in sedimentation. It is proposed 

 to recognize the aggregate importance of such minor breaks by giving 

 them a special name. The name selected is diastem. Diastems range 

 of all values from seasonal cessations of sedimentation to those which 

 approach geologic epochs in riuratiou. 



