GEOLOGICAL IMPORTANCE OF SEDIMENTATION 551 



of interpreting all sediments as marine unless there was positive 

 evidence to the contrary. It is in no spirit of adverse criticism that 

 this is done, but in order to call sharper attention to the degree of 

 variance with the present conclusions and the desirability of con- 

 firming or modifying the latter by further observation and analysis. 



Turning first to the work of the best known of American geologists : 

 J. D. Dana, in his Manual of Geology^ stated that mud-cracks are 

 made on drying mud fiats, but with customary insight was evidently 

 careful not to restrict them entirely to the seashore, since on the next 

 page he refers to them as well as rain-prints as made by "exposure 

 above the water level at low tide, or at least a low stage of the waters. " 

 Thus Dana recognizes the possible continental origin, but places 

 the emphasis, perhaps unconsciously, upon the sea-beach or estu- 

 arine origin. 



Chamberhn and Salisbury in their Geology (1904), Vol. I, p. 466, 

 state that "sediments are sometimes exposed between tides, or 

 under other circumstances, for periods long enough to permit dry- 

 ing and cracking at the surface. " 



Sir A. Geikie in his Text Book of Geology, 4th ed. (1903), pp. 643, 

 644, speaks of mud-cracks as vestiges of shores of former seas and 

 lakes, and one of the kinds of evidence showing that a locahty was 

 sometimes laid bare of water. 



James Geikie, in his Structural and Field Geology (1905), p. 116, 

 mentions the present occurrence of mud-cracks around the shores 

 of inland seas and lakes, and states that the same action may take 

 place on low flat beaches which are exposed to a hot sun during the 

 retreat of the tide. Although lake shores are mentioned first, no 

 discussion is given as to the relative geological importance of the 

 two situations in producing mud-cracks. 



In none of the preceding books is the possibility mentioned of 

 mud-cracks being formed over flood-plains of rivers and apart from 

 permanent bodies of standing water. Yet these authors are author- 

 ities upon the subject of sedimentation and sedimentary structures, 

 and in Sir A. Geikie 's Text Book especially, an appreciation is con- 

 stantly shown of the importance of fluviatile formations. It remains 

 a question, therefore, if this difference of view upon the significance 



I P. 94; see also pp. 742, 745 (1895). 



