TRANSGRESSIVE OVERLAP 727 



rest on the belt of rudaceous deposits of bed h, just as the arenace- 

 ous deposits of that bed come to rest on the rudaceous deposits of 

 bed a. In Hke manner the Uitaceous deposits of bed c come to 

 rest on the arenaceous deposits of bed /;, just as the lutaceous 

 deposits of bed b come to rest on the arenaceous belt of bed a. 

 Two sections, then, made at I and II, will show precisely the same 

 succession in coarseness and kind of material from the bottom up, 

 the only difference being- that in section II the lutaceous bed is much 

 thicker than in section I. Erosion, however, may remove so much 

 of the lutaceous beds of section II as to equalize the amount in 

 the two sections. It would, of course, be incorrect to consider each 

 lithic unit in section I to be of the same age as the corresponding 

 lithic unit in section II, for, although there is a similar lithic suc- 

 cession, bed. a, and the lower portion of bed h are unrepresented in 



Section I. Section II. 



Fig. 144. Diagram showing regular marine progressive overlap on an old 

 land surface. A basal sandstone occurs everywhere, this grading 

 upward and seaward into lutaceous deposits. At section I the 

 series comprises beds b and c only, but at section II beds a-c are 

 present. 



section I. In general, it is safe to assume that in a case of this 

 kind, where continuous transgression on a uniformly shelving shore 

 has taken place, the basal bed of the section farther up the old shore 

 is of later age than the corresponding lithic bed of the section 

 farther seaward. There are many cases where relationships of this 

 type must be considered in the correlation of strata. 



We have so far considered only siliceous detrital material de- 

 rived from an old land composed of crystalline or other siliceous 

 rocks. We must now consider, in addition to these, the organic 

 rocks and their clastic derivations, which play so important a role 

 in the sedimentary series accumulating on the sea bottom. As 

 noted in an earlier chapter, purely biogenic stratified deposits are 

 formed by the accumulation of the various organic oozes on the 

 sea-floor, such as foramini feral, radiolarian, diatomaceous or ptero- 

 podan. Where clastic sediments are accumulating very slowly, 

 as at a distance from shore, these organic oozes luay constitute 

 an important i)art, if not most, of the sediment. In such cases, the 

 ooze being a calcareous one, the clay and other lutaceous beds 



