442 J. BAliKELL KECUG>'1T10^' OF ANOIEKT DELTA DEPOSITS 



Exceptional cases are known where these rules fail, chiefly on account of 

 local accumulations of gravel through proximity to a bold shoreline, but 

 it is thought that they have a high degree of generality. Thinner con- 

 glomerates may be either marine or terrestrial and their mode of origin 

 must be determined on other grounds than that of thickness. The limit 

 which has been rather arbitrarily drawn between sand and gravel by 

 most writers is that of a diameter of two millimeters. For the applica- 

 tion of these rules it should probably be raised to five millimeters. 



Intraformational conglomerates made by wave action on shallow bot- 

 toms and not at the shore are readily discriminated, owing to the local 

 origin and soft character of the pebbles and are not included in this dis- 

 cussion regarding the significance of thickness. 



MUD CRACKS AND RAINPRINTS 



In an earlier series of papers the writer has discussed*^ the relative 

 proportions of continental, littoral, and marine sedimentation and 

 reached the conclusion that deposits of the littoral zone, limiting that 

 term to the land alternately flooded and laid bare at short intervals, are 

 now and always have been small in area in comparison with the areas of 

 marine and continental sedimentation. Furthermore, it was shown that 

 littoral deposits were much more subject to destruction before being in- 

 corporated in the geological record. But mud cracks and rainprints are 

 formed in any argillaceous or calcareous mud when dried either on river 

 floodplains, over playa lake bottoms, and on the shores of eitlier lakes or 

 seas. Of these several situations the river plains, howe\er, offer the most 

 widespread and favorable surfaces for the development and preservation 

 of mud cracks in argillaceous muds. Where the mud-cracked shales 

 possess both a great horizontal and vertical range through formations 

 the evidence is particularly strong for a fluviatile origin, as has been 

 argued in the case of the Mauch Chunk shale of the anthracite coal 

 basins.*^ The same argument applies to the Triassic formations of tlie 

 eastern United States and certain pre-Cambrian formations. Under 

 such conditions of characteristic occurrence through an argillaceous 

 formation it is to be concluded, therefore, that mud cracks form one of 

 the surest indications of continental origin. Several years of observations 

 since these papers were written have served to strengtlien the belief of 

 the writer in this criterion. A few cases of mud cracks in shales, asso- 



*■"' Relative jreological importance of continental, littoral, and marine sedimentation. 

 Journal of Geology, vol. xiv. 1906, p. 550 et seq. 



*^ .Joseph Barrel! : Origin and significance of the Mauch Chunk shale. Bull. Geol. Soc. 

 America, vol. 18, 1907, pp. 449-476. 



