GEOLOGICAL IMPORTANCE OF SEDIMENTATION 549 



coasts where the tides are high, and in this case the areas of sah 

 marsh should be rather frequently cut up by wide and deep tidal 

 channels filled with coarser material. Yet in most mud-cracked 

 formations such erosion of deep channels across the mud-cracked 

 layers is conspicuously absent. 



Under the subject of mud-cracks of fluvial plains the subject of 

 variegated shales was discussed and it was concluded that in those 

 formed under a subarid climate the conditions were especially favor- 

 able for their production. Before closing the present topic, there- 

 fore, the subject should be again mentioned, in order to find if they 

 may not form equally readily in the littoral zone or even in estuarine 

 or open sea portions of the zone of marine deposits. Along the littoral 

 somewhat the same conditions of variable exposure to the air exist 

 as upon flood plains. Below the line of low tide, however, extensive 

 tracts are never exposed to the air except by broad changes of level, 

 and local variations in the amount of organic matter present, to which 

 variegated shales are presumed to chiefly owe their origin, will 

 depend upon conditions of current and influx of sediment. Varie- 

 gated colors should therefore be associated more markedly with 

 variations of texture and composition than is necessarily the case 

 with the deposits upon flood plains; changes more analogous with 

 the contrast between channel sands and true flood-plain muds. 



That red muds as well as blue muds of the terrigenous zone may 

 form on ocean bottoms is indicated by the red muds chiefly found 

 off the Brazilian coast. "Its red color is thought to be due to the 

 great amount of hydrous peroxide of iron brought to the sea by the 

 rivers and which cannot be reduced by organic matter, as in the case 

 of the blue mud. The area covered by it is, however, small, and is 

 estimated at about 100,000 square miles,"^ while the blue muds 

 cover some 14,000,000 square miles. 



Variegated shales may therefore originate in any zone of sedi- 

 mentation and under any depth of water, but those of marine origin 

 are due to broad and slow changes upon the land and should not 

 show any of the local variations and partial independence between 

 color and stratification which may mark the deposits of a flood 

 plain. Within limits variegated shales may be considered, there- 

 fore, as rather characteristic of continental and littoral deposition. 

 I W. B. Clark, Geological Survey 0} New Jersey, Annual Report, 1892, p. 223. 



