GENERAL CONCLUSION 445 



as stumps and roots in place show the fresh-water and continental origin 

 of the beds. As the swamps are in large part, however, on the seaward 

 portion of the subaerial delta beds, a large part of the associated strata 

 is normally marine. This is true, however, of the outer parts of the 

 coal measures only, and on the landward side the whole series may be of 

 terrestrial origin. 



In most of the red bed formations oxidation has destroyed all plant 

 tissues, but in the Catskill of the Upper Devonian and the Mauch Chunk 

 of the Mississippian the writer has found the casts of deep-seated branch- 

 ing rootlets in sitii,'^^ evidence of terrestrial origin of a positive nature, 

 and especially valuable in such formations on account of the usual 

 paucity of evidence. 



The footprints of land animals, and especially herbivorous land ani- 

 mals, are most commonly made on the margins of fresh water. Am- 

 phibians at the present time avoid salt water as a fatal environment, and 

 a similar antipathy has doubtless existed in the past. Footprints are, 

 therefore, evidence of terrestrial origin and fluviatile deposition of the 

 same degree of probability as that furnished by plants. As exceptions 

 to this rule it should, however, be noted that a marine mollusk (Nucu- 

 lana) is preserved on the same slab with the oldest known footprint,^" 

 and footprints of vertebrates in the coal measures of Kansas are pre- 

 served in shales which hold a few marine shells.^^ 



General Conclusion on Ckitehia foe Delta Deposits 



From this review of the criteria which serve to separate the terrestrial 

 ])ortion of delta deposits from those of subaqueous origin several con- 

 clusions may be drawn. First, it is seen that it is more commonly the 

 particular form of a feature, such as cross-bedding or the thickness of 

 conglomerates or the mode of preservation of bones, which is of dis- 

 tinctive value, rather than the mere presence of cross-bedding or con- 

 glomerates or fossils; second, a single criterion is in many cases not 

 absolutely decisive, and it is the convergence of evidence which makes 

 strong the conclusion in regard to the origin of strata of a particular 

 horizon; third, it is unsafe to extend the conclusion beyond the limits 

 of the evidence to other portions of the same formation. Notwithstand- 



*" Origin and significance of the Mauch Chunk shale. Bull. Geol. Soc. America, vol. 

 18, 1907, pp. 460-462. 



^ O. C. Marsh: Amphibian footprints from tho Devonian. American .Journal of Sci- 

 ence, vol. ii. 1896. p. 375. 



"1 O. C. Marsh: Footprints of vertebi'ates in the coal measure's of I\;)iisns, .\mericau 

 .Tournal of Science, vol. xlviii, 1894, p. 81. 



