78 NON-MARTNE FOSSIL MOLLUSCA. 



by the identity of the living with the fossil molluscan types, which has 

 already been referred to. 



Rivers have of course existed ever since a sufficient extent of conti- 

 nental surface was raised above the sea to accumulate the waters that 

 fell from the clouds ; and in view of the mighty changes that have taken 

 place during the progressive growth of the Forth American continent, 

 especially the elevation of its great mountain systems and plateaus, it 

 would be natural to suppose that the earliest rivers at least have been 

 obliterated. Some have no doubt been obliterated, but contrary to 

 what has been the general belief, the recent labors of Powell, Dutton, 

 and others have shown that the rivers of North America have been 

 among the most persistent of its physical features ; that many of them 

 are older than the mountain ranges of the regions which the rivers 

 traverse, and that they have not yielded their " right of way " when the 

 mountain ranges and plateaus were raised, but continued during and 

 after that elevation to run in essentially the same lines which they had 

 chosen when the region they traversed was a plain instead of a mount- 

 ainous one. That ancient river systems have been in some, and per- 

 haps many instances, to a greater or less extent divided, as aconsequence 

 of unequal continental elevation, or from other causes, is quite certain; 

 and it was doubtless in part by this means that the dispersion of fresh- 

 water mollusca into different river systems has been effected. That 

 some formerly existing rivers with their lacustrine portions have been 

 obliterated and their molluscan fauna? destroyed is doubtless also true, 

 but these facts do not necessarily affect the correctness of the view con- 

 cerning the general persistent integrity of rivers and river systems 

 which has been referred to.* 



The coalescence of separate minor drainage systems by the confluence 

 of their lower portions into a common channel during the progressive 

 elevation of the continent has also been an important means of the dis- 

 persion of fluvatile mollusca. By such coalescence, what were once sep- 

 erate rivers or minor drainage systems became parts of larger ones; as, 

 for example, the union of the separate peripheral members of the great 

 Mississippi River system, which now forms a common drainage for the 

 principal part of the continent. The Ohio and Upper Mississippi, the two 

 most ancient portions of the present great system, were once separate 

 rivers, emptying into a northern extension of the Great Gulf; and it is 

 practically certain that neither of them received that portion of the mob 



* The discovery of so few traces of fluvatile deposits as have been made among the 

 strata of the earth is probably due to the persistent adherence of rivers to their an- 

 cient channels. When land upon which rivers have formerly run has subsided beneath 

 the level of the sea, the fluvatile deposits were doubtless destroyed by the encroach- 

 ing marine waters. If the land continued to rise, as has been so generally the case in 

 the gradual production of the North American continent, the earlier river deposits 

 were swept away in later times by their own waters, as their valleys were broadened 

 and deepened. It is therefore, as a rule, only in (lie deposits of lacustrine portions of 

 ancient river systems that tueir faunae have been preserved. 



