336 E. C. CASE CLIMATIC ENVIRONMENT OF EXTINCT ANIMALS 



different from that of sediments derived from a land of softer rocks. 

 Obviously coarse, angular, or semiangular grains or pebbles, loosely piled 

 and with the original minerals little changed, tell of rapid erosion and 

 transportation by active streams, and if such beds are recurrent at fairly 

 regular intervals the suggestion is of a humid climate with regular alter- 

 nations of wet and dry ^seasons. If similar beds occur sporadically, the 

 suggestion is one which will occur to every observer who has worked in 

 an arid or semiarid region, where infrequent but violent storms occur 

 between long intervals of drouth. 



If the material derived from a land of hard rocks is thoroughly decom- 

 posed, it tells of a land where the eroded material lay long as a part of 

 the residual soil in a climate of at least sufficient humidity to permit 

 active chemical action by COo derived in large part from decaying vege- 

 tation. 



If the sediments were deposited on the littoral of a lake, on a great 

 delta, on a river floodplain, or in the bed of an overloaded stream, their 

 chemical composition and the nature of the cement gives a good idea of 

 the prevailing height of the water-table, and this in turn reveals the 

 general condition of humidity at the time. AVith a high water-table, the 

 deposited minerals and the cement will be largely sulphides, hydrates, 

 and the lower oxides ; with a low water-table, there is possible a circula- 

 tion of air and oxygen-bearing waters, and the minerals will be sulphates, 

 carbonates, higher oxides, and even Avater-soluble salts in large quantity. 

 The condition is revealed in part in the coloration and the texture of the 

 beds as well as in the minerals themselves. The minerals of the latter 

 series are less bulky and less firm in mass than the former and permit the 

 presence of cracks and cavities. 



Altered Sediments 



If we assume the more difficult alternative, that the sediments have 

 been altered during the long time since their deposition, we must reckon 

 with the profound changes due to the infiltration of migrating waters or 

 the drainage of the beds and the consequent changes due to oxygenation. 

 Such changes are not usually so profound as to obliterate all traces of the 

 original condition, and so the process may be detected. The migration 

 of waters into sediments laid down under arid conditions tells their story 

 in the presence of casts of crystals of water-soluble salts or pseudomorphs 

 after such crystals, in the increase of bulk by hydration and the conse- 

 quent disturbance of the beds, and in the deposition of minerals in the 

 original cracks and cavities. Drainage and desiccation of the beds, origi- 

 nally water soaked, leaves its own peculiar record. 



