INTRODUCTION 



have been covered with sediment brought by water. Deposits 

 upon the surface of the continent — lake, flood plain, alluvial- 

 fan and playa deposits — are in the zone where erosion is domi- 

 nant and are hence apt to be quickly, geologically speaking, 

 worn awav with all their included organic remains. Those 



Fig. I. — Diagram to illustrate that the habitat of a plant or animal determines 



its chance of preservation as a fossil. 



deposits are thicker and stand a much better chance of preser- 

 vation, which are poured by continent-draining rivers into the 

 sea as a delta and are spread upon its margin as a flood plain 

 (Fig. i). The delta and flood plain of the Ganges and Indus 

 rivers (Early Tertiary to present) and that forming the Mauch 

 Chunk shales (Mississippian) of Pennsylvania are good examples. 

 The seaw^ard portion of these deposits, being more continuously 

 under water, is best able to preserve the organic remains de- 

 posited in it ; hence we find the Greenbrier formation, inter- 

 fingering the Mauch Chunk shales in southwestern Pennsylvania, 

 full of marine fossils. The landward portion is covered with 

 water and its accompanying layer of sediment only during 

 times of flood when the river overflows its banks ; during the 

 rest of the year the level of the ground water sinks to a greater 

 and greater depth, gradually dissolving all soluble objects as 

 it moves, while the air circulates freely through the upper water- 

 free deposits, completing the oxidation of all organic remains 

 there present. The decay of plants or animals is an oxidizing 



