GEOLOGICAL IMPORTANCE OF SEDIMENTATION 539 



familiar examples.' In such regions the conditions of mud-crack 

 formation are at a maximum and may extend to the margin of the 

 littoral zone.'' Consequently carbonaceous deposits and mud-cracks 

 both mark the land surfaces of aggrading rivers, the one a maximum 

 in pluvial climates, the other in arid. Mud-cracks, as contrasted 

 with coal beds, may thus serve as an index to ancient climates as well 

 as possessing a stratigraphic significance. 



In applying this distinction to the earlier geological periods it 

 should be held in mind, however, that there is no evidence of an 

 arboreal or even herbaceous vegetable covering to the land previous 

 to the Silurian, and its surface was presumably devoid of life save 

 possibly that of the lowest cryptogams. Under such circumstances 

 the indications of the presence of former flood-plain surfaces by 

 means of carbonaceous deposits or deoxidizing effects upon the ferric 

 oxide might be entirely absent. Mud-cracks would be the safest 

 remaining indication of the flood-plain nature of the land surface 

 over the regions where the character of the detritus was suitable 

 and periods of desiccation were sufficiently long for the formation 

 of the cracks and hardening of the successive surface layers. 



The necessary fineness of deposit is frequently not found on the 

 sandy or gravelly fans of mountain streams, and hence a large per 

 cent, of stream-built deposits could not be expected to show this 

 feature. The necessary conditions are found, however, on all 

 streams of small gradient which broadly overflow their channels, 

 this being characteristically the case of the larger rivers in the lower 

 portions of their courses and especially over the delta, where the 

 argillaceous nature of the deposits is well known. Broadly speaking 

 then, the formation of mud-cracks is non-essential on slopes of 

 piedmont river waste, but is especially characteristic of the larger 

 river plain and delta deposits of arid and subarid climates. That 

 the phenomenon is not strictly confined to even subarid climates is, 

 however, true since humid climates may have their seasons of 

 dryness. 



1 See for illustration, Daniel Trembly Macdougal. "The Delta of the Rio Colo 

 rado" (with map by Godfrey Sykes), American Geographical Society, Jan. '06. 



2 Walther, Das Getz der Wiistenbildung, 1900, also mentions the occurrence of 

 mud-cracks on arid flood-plains. See Trockenrissen in index. 



