WORK OF JOSEPH BARRELL OX SEDIMENTATION 37 



"It would seem that next to coal beds formed in situ, or abundance of land 

 fossils belonging to the animal kingdom, that mud-cracks form one of the 

 surest indications of the continental origin of argillaceous deposits." ^^ 



The significance of mud-cracks, and also of rain-prints and rootmarks, 

 is entirely convincing when developed, both broadly and vertically through 

 mechanical sediments/' ^^ 



From a study of the conditions most favorable for the production of 

 thick conglomerate formations/^ Barrell concluded that continental con- 

 glomerates exceeded marine conglomerates in importance. He says : 



"A maximum limit to widespread basal marine conglomerates seems to be 

 100 feet, and therefore broad conglomerate formations of greater thickness 

 are evidence of terrestrial accumulation." ^^ 



Minor features indicating marine or river action is pointed out. For 

 instance, he says : 



"W. D. Johnson pointed out that in the Tertiary deposits of the High Plains 

 the gravel courses, where exposed to observation, are greatly elongated in the 

 direction of the streams— that is, in the direction leading away from the source 

 of supply. Mansfield has noted that shore gravels, on the contrary, are ex- 

 tended in courses parallel to the margin of the deposit." 



He also says : 



"River gravels are shingled by the currents, so that the longer diameter of 

 the pebbles dip upstream, giving a faint appearance of false bedding, which 

 on the average, unlike the false bedding of sandstone strata, dip toward the 

 basin margin. Shore gravels, on the other hand, are developed parallel to the 

 shore. The onshore waves have a greater force than the undertow and the 

 shingling dips away from the shore, or runs out laterally from protruding 

 headlands." ^^ 



The color of rocks, particularly with reference to the continental de- 

 posits, was discussed in considerable detail by Barrell. He says : 



"Such a relation of red shales and gray or green sandstones may then be 

 taken as presumptive evidence of subaerial deposition. It should not. how- 

 ever, be taken by itself as positive evidence, as the number of cases studied on 

 which the conclusion rests is still somewhat limited.'" 



And regarding lateral and vertical variegations in clays, he says : 

 "Such variegated beds are then highly suggestive of terrestrial deposition, 



1^ Op. sup. cit., p. 550. 



"Am. Jour. Sci. (4), vol. 36, 1914, p. 438. 



1^ Some distinctions between marine and terrestrial conglomerates. (Abstract.) Sci- 

 ence, n. s., vol. 29, p. 624 ; Bull. Geol. Soc. Am., vol. 20. 1910, p. 620. 

 "Am. Jour. Sci. (4), vol. 36, 1914, pp. 439. 440. 

 19 Bull. Geol. Soc. Am., vol. 27. 1916. p. 357. 



