358 ORTMANN— THE ALLEGHENIAN DIVIDE. [April 18, 



tion of these streams in consequence of a reelevation of the coun- 

 try. According to Foshay (1890, p. 400) and others, this rejuvena- 

 tion is of Postglacial age. Thus ive might expect to find in these 

 upper parts of the mountain streams, the remnants of the fauna 

 zvhich existed in these rivers in Preglacial {Tertiary) times. I have 

 no doubt, that at least some of these are Tertiary elements, and pos- 

 sibly just those which are found on either side of the mountains 

 might belong to them. However, this fauna is too fragmentary, 

 to be sure about this, and it is quite evident, that also in Tertiary 

 times not the whole of the fauna of these rivers went up to near 

 the headwaters. Thus we have to wait till additional evidence with 

 regard to the Tertiary fauna of the headwaters of the Erigan sys- 

 tem is forthcoming.^^ 



Fact II., i, (a). 



It has been seen, that there is a certain amount of uniformity in 

 the Atlantic fauna, in spite of the fact that the Atlantic river sys- 

 tems are quite isolated from each other. In fact, most of the At- 

 lantic species are not restricted to a single drainage, but are found 

 in several, often practically in all of them. This means, that there 

 is or there was the possibility of an intercommunication of the 

 faunas of these rivers, and the question arises, how this was 

 brought about. 



All these rivers, after having traversed the Piedmont Plateau, 

 run for a greater or lesser distance through the Coastal Plain. This 

 plain is little elevated above sea-level, and consequently the rivers 

 are sluggish here; there is considerable deposition of material in this 

 region, and a great tendency toward a change in the river channels : 

 the rivers are practically at base-level. It is a general rule, that in 

 a country approaching base-level, the intercommunication of neigh- 

 boring rivers is greatly facilitated (see Adams, 1901, p. 842), and 

 that consequently a wide distribution of the fauna is favored. 



^° The best evidence would be fossil forms from the high river terraces. 

 Such do exist, but the remnants are too poorly preserved, to be of any 

 value. It should also be noticed, that there is a number of species in the 

 upper Ohio drainage, which distinctly avoid the larger rivers : also these 

 might be elements of the old Tertiary fauna. It is interesting, that several 

 species of the present fauna of the mountain streams fall into this class, 

 namely: Symphyiiota costata, Alasmidonta marginata, Sirophitiis edentulus. 



