416 J. BARRELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



cited in the beds which commonly underlie the Cambro-Ordovician lime- 

 stones; also of many Upper Paleozoic sandstones, which because of asso- 

 ciation with fossiliferous beds and distance from the sources of erosion 

 imply a marine origin. 



In mudstones of continental origin absence of fossils is especially asso- 

 ciated with red beds, implying a completeness of oxidation and solution. 

 Carbonaceous shales of land origin if of post-Silurian date commonly 

 preserve some plant remains. In marine beds, however, carbonaceous 

 shales are not so favorable to the existence of fossils, apparently owing 

 to the absence of plants with fibro-vascular tissues and of stagnant bot- 

 tom conditions. Calcareous shales of continental origin are more apt to 

 be barren of fossils than marine shales of the same composition. 



Skillful search and the accident of exposure have resulted, however, so 

 frequently in finding rare fossils or special kinds of fossils associated 

 with apparently barren formations that absence of fossils in connection 

 with the kind of beds should be used merely as a criterion suggestive 

 and not conclusive of the mode of origin. 



COLOR OF SEDIMENTS AND THE RELATIVE INFLUENCE OF LOCATION AND 



CLIMATE THEREON 



Sediment carried by rivers is subjected to oxidation both while in 

 transit and after deposition on the surface of the floodplain, until its 

 burial by overlying layers carries a stratum below the level of ground 

 water. Where the ground water level is coincident with or higher tlian 

 the surface, organic matter accumulates and deoxidizing processes take 

 place. A certain fraction of delta deposits, depending on the proportion 

 of back swamps and coastal swamps, therefore show colors ranging from 

 green to blue, according to the state of the iron oxide, and from white 

 through gray to black, according to the amount of carbon. But over the 

 larger portion of the delta the iron of the soil is more or loss com])lctely 

 oxidized during the seasons of dryness, and lli(> con-cspoiiding colors — 

 3'elloAv, orange, red, or brown — are in rvidcnce. 'V\\c ratio of llipse oxi- 

 dized and deoxidized sediments varies with the ilatness of the delta and 

 the character of the climate. Color is therefore in itself no criterion by 

 which to distinguish between terrestrial and subaqueous deposition, but 

 yellows, reds, and browns are the dominating colors of continental de- 

 posits save in certain geological periods. A red shale which grades later- 

 ally into a green, gray, or black shale gives therefore strong indications 

 of terrestrial origin for the red portion. Such a relation is found in the 

 Wamsutta Red Beds of the Rhode Island basin. The other portions may 

 be swamp deposits of deltas or, so far as the color goes, of subaqueous 

 origin. This evidence from red beds is especially strong when found in 



