412 J. BARRELL INFLUENCE OF CLIMATES ON VERTEBRATES 



Here, with the change in the nature of the sediment, the record reverts 

 again to one of footprints and not of bones. 



The significance of this has been discussed by the writer in other 

 papers. 26 The amphibian footprints and bones are found in terrestrial 

 deposits. The footprint of Thinopus, it is true, was made in a muddy 

 sand holding a small marine gasteropod, Nuculana. Here, on the outer 

 margin of a delta, the sea had contributed to the material, but the asso- 

 ciated strata show dominant delta conditions, and, in the wide oscilla- 

 tions of the strand-line characteristic of delta fronts, deposition under 

 shore and river conditions alternate. In the Mauch Chunk shale of east- 

 ern Pennsylvania, on the contrary, there is no trace of the sea, and here 

 the amphibian footprints were left in drying muds in the midst of a 

 broad delta plain, far removed from permanent waters. 



The red shales and sandstones are markedly barren of organic remains, 

 yet footprints and plant impressions are present. The sediments were 

 characteristically deposited under conditions where they were subjected 

 to drying and atmospheric oxidation. The recurrent drying out implies 

 a fall of level of the ground-water. Such changes in ground-water, 

 through the induced circulation, favors solution of slightly soluble ma- 

 terials, such as the mineral matter of bones, in the zone above. Even 

 large and resistant bones are speedily destroyed if alternately wet and 

 dried in the presence of oxygen and seeping waters. Such conditions are 

 present in the delta soils of seasonally arid climates, but not in wind- 

 formed desert deposits nor in the swamps wherein organic matter accu- 

 mulates. The wetter and cooler the climate the more favorable become 

 the conditions for the spread of swamp conditions, resulting in the accu- 

 mulation of coal and permitting also the preservation of animal fossils. 



A nearly continuous record of continental floodplain deposits may be 

 followed from the close of the Silurian to the close of the Paleozoic and 

 discriminated from the synchronous marine formations. This record 

 shows a recurrent and increasing diastrophic and climatic instability, 

 dating from the beginning of the Devonian. The record of climatic 

 oscillation is seen in the manner in which the dominantly gray to black 

 deposits of the Middle Devonian in the Appalachian basin give place in 

 the Upper Devonian to the gray to olive marine shales of the Chemung 

 and the corresponding red shales of the terrestrial phase known as the 

 Catskill, indicating a movement toward aridity, though not attaining 

 actual desert conditions. Then, at the opening of the Mississippian, a 

 swing of the climatic pendulum toward wetter and possibly cooler condi- 



26 In relation to the amphibia see "The origin and significance of the Mauch Chunk 

 shale." Bull. Geol. Soc. Am., vol. 18, 1907, pp. 449-476; especially pp. 472-474. 



