8 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



of the channel form stretches of swamp and morass, where abundant terrestrial 

 vegetation would result in the formation of highly carbonaceous beds. 1 



Tidal marshes, lagoons, and bayous subject to occasional or periodic 

 inundation by the sea would also present a mingling of brackish-water and 

 marine forms of life, but the deposits would be more regularly bedded and, 

 due to the greater intervals between periodic floodings, there would be a 

 zonal distribution of the fossils with a recurrence of the distinctive faunae. 

 This theoretical arrangement might be disturbed by the action of storm- 

 waves in shallow water and perhaps difficult to detect because of the thin- 

 ness of the successive deposits. In general, however, the deposits would 

 be more regular and more easily denned geographically than those of an 

 estuary. Mapping should give a hint as to the nature of the deposit, as the 

 linear form of the estuary would generally betray its character if considered 

 with the other features. 



FRESH-WATER DEPOSITS. 



Lakes of large size reproduce in some degree the form of coastal and 

 shallow-water deposits of the oceans, but the presence of fossils of fresh- 

 water life would at once distinguish the two. Due to the smaller size, 

 lack of tides, strong currents, and powerful wave-action, there would be less 

 perfect sorting of the material; i. e., the relatively far greater amount of 

 terrestrial material swept in would always be a noticeable factor, and no 

 surprise would be felt at the appearance of the remains of land animals 

 or plants. 



SUBAERIAL DEPOSITS. 



Fluviatile deposits are most apt to be encountered as seemingly extraneous 

 accumulations in homogeneous beds or, if making up the bulk of the observed 

 beds, as irregular masses difficult to resolve into their constituent elements. 

 When recognized and analyzed into parts, each deposit is linear in general 

 form if enough remains for the shape to be made out, or if the conditions are 

 favorable for such an observation. Not uncommonly river deposits are 

 revealed in cross-section by the dissection of old plains, flats, swamps, or 

 even marine deposits. 2 In such cases the deposits were generally laid 

 down in more or less sharp valleys formed in an uplifted and eroded marine 

 deposit. Alluvial fans formed either upon land or in shallow bodies of 

 water of small size also belong in this group. True deltas are considered 

 elsewhere. 



Old river-channels are generally readily recognized by the shape of the 

 cross-section of the deposits and the arrangement of the material as a linear, 

 narrow lens quite sharply marked off from the material on either side. 

 In such deposits the inorganic material and fossils, other than the aquatic 

 forms directly referable to the river itself, would be apt to resemble those 



1 Folios 86, 106, and 139, U. S. Geologcial Survey. 



1 Case, E. C., Carnegie Inst. Wash. Pub. No. 207, p. 78, 1915. 



