RHYTHMS IN SEDIMENTATIOiST 781 



The depths of Avatei' given for these examples are probably somewhat 

 above the a^'erage for their types, since tlie ancient examples were com- 

 monh' shallow throiighont and were generally not provided with adjacent 

 reservoirs into which the excess of sediment conld be readily swept. In 

 such shallow bodies the wave action is damped down by the energy ab- 

 sorbed in stirring the bottom and can not become progressively gj.'eater 

 with breadth of the ^vater l)ody. During storms choppy wave action and 

 sediment churned up with water are the results." Upon the sul)sidence of 

 the wind the sediment, even if mnd, quickly settles in salt waters. Sedi- 

 ment must be slowly M'orked to the region of exit and escapes chiefly by 

 combined wave and cirrrejit action in time of storms. If the connections 

 with the outer ocean art; restricted, the water of the l)asin is shallower 

 than for an open water Ijody and a. greater degree of l)ottojn stii'iing will 

 occur during storms. An average depth of several inches of sediment 

 may thus be lifted from the bottom during the exceptional storms, to be 

 cpiickly deposited during the following calm. 



When evaporation exceeds rainfall in a nearly enclosed basin, such as 

 the Mediterranean or Eed Sea, the currents in general flow inward instead 

 of outward. In extreme conditions of aridity sodium chloride is deposited, 

 in less extreme conditions gypsum is the chemical precipitate; but even 

 gypsum requires for its precipitation the concentration of present sea- 

 water to 19 per cent of its original volume. Calcium and magnesium car- 

 bonates are insoluble, however, in sea-water concentrated one-half, or with 

 lesser concentration where a lime mud has saturated a normal sea-Avater. 

 In lime-depositing seas any tendency for evaporation to exceed rainfall 

 in partly inclosed water bodies may tlms supph' more sediment to the 

 Ijottom than is swept out through the restricted exits. The water body 

 thus grows shallower until a seasonal change in wind may alternateh' 

 cover and lay bear ^ast flats of lime muds. The importance of this action 

 is indicated in the past by the abundance of intraformatioual conglomer- 

 ates, mud-cracks, and occasionally rain-prints in certain limestoues, as, for 

 example, in the Precaud)rian rlolomitic limestones of tlie Beltian system, 

 in many Cambrian and OrdoAiciau dolouiitic; liinestones, and in ttie Avater 

 limes of the Salina epoch in eastern NTortb America. Thus have 1)een 

 developed the play a seas. 



Where sedinu'ui is ])ourc(l al)uu(hrntly into an epeiric sea, the water is 

 someAvhat shallowed for a long distauce. For example, soft mud is the 

 chief material brought by the Tigris ajid Euphrates to the Persian Gulf 

 and a depth of ten fathojus is reached only at a distance of 40 to 45 

 miles from the head of the gulf. The character of sediment on all 

 bottoms tends to varA- u'ith depth, the sand being kej^t in the shallower 



