212 Evolution and Distribution of Fishes 



As we now have to deal biologically with the two great 

 sets of freshwater and marine fishes, as well as some that 

 seem to have been more or less anadromous in habit, the 

 preceding chart may aid in correlation of the beds dealt 

 with in the countries where the Cretaceous is most carefully 

 studied. But it must be understood that the correlation is 

 an attempted effort toward scientific exactness, but may 

 be in time considerably modified. 



Beginning with the lowest beds that make up the Weal- 

 den formation, this has generally been conceded as of 

 freshwater origin But Geikie (7:11:940), Chamberlin 

 {8: III: 128), and others have regarded the entire English 

 deposit as having been laid down in a large "delta 20,000 

 or 30,000 square miles in extent." Many considerations 

 however militate against this. Thus delta deposits are 

 typically irregular, interbedded, and devoid of uniform 

 organic strata, those of the Wealden are largely clays that 

 indicate steady precipitation in still waters, and which have 

 definite organismal zones; delta deposits are restricted to 

 narrow and irregular channels which are interrupted by 

 land masses between, those of the Wealden are extended 

 and agree surprisingly well with those of the American 

 Potomac, as well as of the Weald of North Germany. 



The writer would therefore favor the explanation given 

 by C. J. A. Meyer in his paper "On the Wealden as a 

 fluvio-lacustrine formation" {i^S- 243). He adduces good 

 evidence for the view that the freshwater Purbeck beds are 

 uniformly continuous with the base of the Wealden; that 

 the Wealden strata were mainly laid down in shallow lakes 

 that seem at times to have dried up over considerable areas 

 so as to cause sun-cracks; that such remains as fruits of 

 Chara, in addition to the abundant fern, equisetum, cycad, 

 coniferous fossils previously recorded, also the varied 

 invertebrate organisms like Cypris, Unto, Physa^ Pahidina, 

 and Planorbis^ likewise the freshwater fishes and amphib- 

 ians already known all indicate a fluvio-lacustrine environ- 

 ment. He therefore concludes: 



(i) That the Wealden strata are a fluvio-lacustrine 

 rather than a purely fluviatile or fluvio-estuarine deposit. 



