210 GEOLOGICAL DISTRIBUTION. 



should have accumulated in these river deposits in considerable 

 numbers; and, when once there, there is no reason for supposing 

 that their destruction would have been very much, if at all, more 

 rapid th£:n in the case of marine shells. But, if none of the forms 

 that occur associated with distinctively marine types are in reality 

 of a fresh-water nature, what has become of these remains ? Surely 

 it cannot be that they have suffered complete, or even nearly com- 

 plete, destruction. It seems far more reasonable to assume that 

 many of the forms described as marine, from indisputably oceanic 

 deposits, are in reality not such ; and this may be the case not only 

 with such forms as, in their generic characters, are but barely dis- 

 tinguishable from known fresh-water types, but even with those 

 which have a strictly marine facies. For there can be little or no 

 question that the primitive fresh-water fauna was a derivative from 

 the marine ; hence, the earliest fresh-water tyjies must have been of 

 a structure but little different from that of their oceanic progeni- 

 tors, and barely, if at all, distinguishable by external characters 

 from them. From the first, however, they will have been subject- 

 ed to the modifying influences which result from a change in the 

 physical conditions of the environs, and which have wrought in the 

 course of ages (and rendered more or less permanent) those structu- 

 ral features which, at the present day, serve to distinguish the ma- 

 rine from the fluviatile type of organism. 



If direct proof of the ready adaptability of marine or brackish- 

 water organisms to fresh -water conditions, or the reverse, were 

 needed, no more decisive testimony in this direction could be had 

 than is furnished by the ancient lake region of the "Western United 

 States, which marks the position of the Laramie or Lignitic forma- 

 tion. The lacustrine deposits of this formation, which attain a 

 maximum development of some four thousand to five thousand feet, 

 have evidently been laid down in lake-basins formed through the 

 land-locking and slow desiccation of a continental arm of the sea, 

 which projected completely across the United States during tlie 

 Cretaceous period. The exclusively marine character of the organ- 

 isms which flourished at this time clearly indicate what was the 

 condition of the waters which they inhabited. Through a gradual 

 elevation of tlie land, which appears to have set in about tlic close 

 of the Cretaceous period, and the consequent formation of barriers, 

 the waters of this vast inland sea, by reason of their severance from 



