782 J. BARRELL MEASUREiMEiNTS OE GEOLOGIC TIME 



water^ but by far the most abundant sediment supplied by rivers is mud. 

 Consequently, even shallow waters receiving such sediment show mud 

 l3ottoms with fine sand as a subordinate constituent. The nature of the 

 bottom of epeiric seas is consequently more dependent upon the breadth 

 of water body, intensity of wind action, and character of sediment than 

 upon the depth of water. 



Schuchert, on the basis of the depth of wave action on the exposed 

 shelves facing stormy oceans, has divided the seas into littoral seas and 

 deep water pelitic seas, the littoral seas being less than 150 feet deep, 

 the pelitic seas being from 150 to 600 feet in depth. -^ In view of the 

 previous discussion it would seem that this, although applying to the shelf 

 seas of the present, would be an overestimate for the epeiric seas of the 

 previous eras. At times of world-wide equability of climate the tempera- 

 ture gradient bctw^een the latitudinal zones was weakened and the mean 

 intensity of winds must apparently have been less, although the atmos- 

 pheric conditions for the spread of tropic warmth to polar latitudes was 

 at such times more efficient. In the epeiric seas, especially of the Paleo- 

 zoic, the conditions of wave action should have approached those now 

 found in shallow, partly land-locked bodies of water in the tropics. The 

 prevailing wave bedding of the ancient deposits shows that the water was 

 seldom deeper than wave-base. The general conditions of depth were 

 therefore those shown in the previous tabulation for quiet epeiric seas, 

 lagoons, and playa seas. It may be concluded that the ancient seas were 

 typically not more thair 100 feet in depth in their central parts and were 

 often only a fraction of this depth over tens of thousands of square miles. 

 Less frequently a rise of sealevel faster than sediment could fill in a basin 

 would give depths of 200 or even 300 feet, but seldom deeper. Iicviewing 

 the prevailing conditions through geological time, it is scon that wave- 

 base is a surface relatively parallel to sealevel and but little below it.. 

 During a stage of prolonged crustal quiet the wave-base is a surface below 

 sealevel comparable to the lluviatile baselevel of the lands above sealevel, 

 the former being depressed generally not more than 100 to 200 feet below 

 the other, the one passing into the other through a slightly accentuated 

 flexure constituting the shore and near-shore zone. 



Thus baselevel may be used as a wide and inclusive term, applying 

 both to land and sea. jSTearly all sediment on the continental platforms, 

 either terrestrial or marine, is deposited with respect to it. A further 

 fundamental feature consists in the oscillation in short and long periods 

 of this baselevel surface. On the land, in climates jjermitting lluviatile 



-4 Pifsson and Schucbeut : Text-book of Geology, 1915, pp. 490-498. 



