GEOLOGICAL IMPORTANCE OF SEDIMENTATION 431 



open coasts, 100 fathoms is the average depth at which fine mud or 

 ooze commences to form; but local exceptions may be noted. In 

 general, the cleaner sands are restricted to a belt within 30 miles of 

 shore, and the average limit of the blue muds is about 200 miles, 

 though this varies greatly in different regions. The relation of these 

 terrigenous to the true pelagic deposits is well brought out in Chart 

 I, Deep Sea Deposits, Challenger Expedition. This area of marine 

 deposits of land-waste is etimated to cover about one-seventh of 

 the globe and to be equal to half the area of the continents, while 

 the area within the loo-fathom line, which more immediately con- 

 cerns the present subject, is estimated at 10,000,000 square miles, 

 or about one-fifth of the continental areas. 



The deposits of former ages corresponding to the shallower and 

 deeper portions of the terrigenous zone may be distinguished by a 

 number of textural and structural features. For instance, consider- 

 ing the conditions of present deposition, it is seen that the ancient 

 equivalents of the present blue muds will be rather widely extended, 

 uniform, massive, argillaceous deposits, usually slightly carbonace- 

 ous and grading into calcareous formations. They will be without 

 cross-bedding and ripple-marks, since movements sufficient to form 

 these would prevent the settling of muds. They may be distinguished 

 from ancient estuarine muds by all these features, and especially 

 the absence of frequent alternations of sand. It is doubtful, however, 

 if the deeper portions of these blue muds have ever been elevated into 

 land surfaces. On the other hand, the shallower belt of marine 

 deposition will be marked by a sandy character of deposits, provided 

 that the rocks of the land can furnish sand, by evidences of shifting 

 current-action and by ripple-marks. A. R. Hunt has shown that 

 the waves of storms may stir the sands of the bottom to a depth of 

 40 fathoms sufficiently to move gravel and injure living molluscs.^ 



The charts of the United States Coast and Geodetic Survey also 

 show sand and comminuted shells to similar depths, from which it 

 may be inferred that widespread arenaceous and unfossiliferous 

 formations may be formed on the bottoms of shallow seas, and not 

 necessarily in close proximity to the littoral zone. 



I "On the Formation of Ripple Marks," Proceedings of the Royal Society of 

 London, Vol. XXXIV (1882), p. i. 



