RHYTHMS IN SEDIMENTATION 777 



in even layers and are often ripple- or current-marked, indicating the 

 wide-spread and nniform action of waves on a shallow bottom. Flooclplain 

 sands show less continuity and regularity of deposit, indicating a preva- 

 lence of shifting currents over deltas and basins. 



It may readily be granted that the coarse deposits are of shallow-^A^ater 

 origin, but the question of the depth at which the fine-grained sediments 

 are on the average deposited is of more importance. Mud is tlie most 

 abundant sediment, and shale consequently must be the most abundant 

 rock. A not inconsiderable portion, as is now recognized, has been de- 

 posted on river floodplains. Eain-prints, mud-cracks, or other features 

 bear evidences of subaerial exposure and indicate generally this mode of 

 origin. Marine muds where intercalated with layers of sand indicate 

 that they are shallow-^^-ater deposits, since the sand can be transpoited 

 (mly in limited depths of water. "Where the interstratiiied sands are thin 

 and regular, this feature indicates spreading by Avave action and the sedi- 

 ment Avas deposited at A\'a\'e-base. In certaiji formations, on the other 

 hand, in contrast to this interstratified type of mud and sand deposit, both 

 sand and the marks of exposure to the air are absent, t1ie mud consisting 

 of an impalpable materia], occurring in tliin lamina^ Avlvich may be paper- 

 thin, and varying in color rather than in texture from stratum to stratum. 

 These are presumably lake or sea deposits made in quiet water below wave- 

 base. Such may be classified as deep-water deposits, but among the shales 

 they are the exception rather than the rule. 



Limestones are taken as evidence of clear seas. They have often beer 

 regarded as deposited in deep Avater, but, on the contrary, most of tb( 

 limestones Avhich are exposed on the continental platforms shoAV l)y tlieii 

 structures that they were deposited in shallow Avater. In the older geo- 

 logic periods the contributions of the cryptozoa, bottom-living al^^, indi 

 cate that the bottom was effectively lighted by the sun. Mud-cracks and 

 rain-prints at many horizons in limestones prove recurrent exposure to t1i( 

 air, alternating Avith submergence in shalloAv Avaters, usually marine. lu- 

 traformational conglomerates are frequent in calcareous, and dolomiti(- 

 formations. They OAve their existence, in at least considerable degree, tc- 

 the drying and cracking of limestone crusts deposited by cryptozoa oi 

 deposited as lime silts Avhich were cemented near the Avater surface at oi 

 immediately after accumulation. The fragments have been rounded and 

 further broken by Avave action. The chalk Avhich is exposed to observa- 

 tion Avas formerly regarded as -an abyssal ooze, luit is now commonly be 

 lieved to have been de])osited mostly in shalloAv seas. The evidence is 

 found in tlie stout-shelled moljusks which occur as associated fossils, 

 'i'hese thirk-shf'Il«^d typps represent adaptations to relatively shallow Avater : 



