EVALUATION OK STRATKJltA I'll IC CIUTKKIA 443 



ciatecl with a marine or brackish water fauna, have been noted, but they 

 are rare and limited in development. 



In regard to mud-cracked limestone.s, however, the case is quite differ- 

 ent. They are hardly to be explained by modern worhl conditions, and 

 in 1906 they were purposely excluded by the writer from the discussion 

 on mud cracks, as not sufficient data had then been accumulated to treat 

 them. Since then numerous observations have shown the common occur- 

 rence of mud-cracked limestones in a number of Paleozoic formations, 

 and leads to the conclusion that the seas of those particular epochs were 

 essentially marine playas, extremely shallow pans of sea water. These 

 were repeatedly emptied, not by the rapidly recurrent ebb and flow of 

 tides, but possibly by monsoon winds or at longer intervals by extremely 

 slight changes in the relative elevation of the playa bottom with respect 

 to the sealevel. A related feature necessary to postulate in order to 

 permit of such a condition was that the lands were so low or the rainfall 

 so light that land waste and fresh water were not supplied in large quan- 

 tities to these basins and the lime sediment was carried into them in 

 solution by the sea. Land waters supplying almost wholly material in 

 solution may also have contributed to certain formations. If waste had 

 been supplied by the land in considerable quantities subaerial deltas 

 would more or less completely have displaced the abnormally shallow sea 

 and mud-cracked argillaceous deposits of continental origin would have 

 been laid down. 



TERRESTIilAL FOSSILS AS EVIDENCE OF TERRESTRIAL DEPOSITS 



Free terrestrial fossils, plants, and animals. — Marine organisms are 

 not washed inland by any usual process, but, on the contrary, rivers 

 may carry river and land dwelling forms into lakes or seas. The prob- 

 lem is raised, therefore, To what extent are free terrestrial fossils safe 

 criteria of terrestrial deposits? Xo discussion will be given here of what 

 groups are to be safely regarded as terrestrial, but rather granting a 

 terrestrial nature. To what extent may their remains become entombed 

 in the deposits of permanent water bodies? 



As to plants — trunks of trees and coarse vegetation are carried to sea 

 in abundance by all large rivers whose channels are bordered by such 

 plant life. Delicate parts, such as fronds of ferns and leaves, can not, 

 however, be carried many miles without maceration, and their perfect 

 preservation in abundance argues for the presence of swamps or small 

 lakes. 



In regard to the limitations in the occurrence of the bones of terrestrial 

 animals, the most definite observations have been made by Hatcher. The 



