I9I3.] ORTAIANN— THE ALLEGHENIAN DIVIDE. 349 



of the country and the direction of the edge of the ice made them 

 find this outlet in a southwesterly direction, thus connecting the old 

 Preglacial systems by a new river, which was the beginning of the 

 present Ohio. The Ohio thus was formed during Glacial times. 



The northward flowing Preglacial rivers were connected by a 

 master stream called Erigan River, running in a direction about par- 

 allel with the direction of the present St. Lawrence. There is some 

 dispute as to the direction of this old river (northeast or southwest), 

 but the evidence preponderates which assigns to it a northeasterly 

 flow. The present writer has shown also (1906, p. 429) that cer- 

 tain facts in the distribution of crayfishes point to this conclusion, 

 that is to say, that this drainage finally was eastward into the At- 

 lantic Ocean. This question will be discussed farther below. 



E. Mutual Connection of the Atlantic Streams. 



The present Atlantic streams, Delaware, Susquehanna, Potomac, 

 James, Roanoke, are quite independent from each other, and dis- 

 charge separately into the sea, so that no direct intercommunication 

 of their waters seems possible. However, we have seen that their 

 headwaters interlock closely, and that it is probable that in the past 

 stream capture has taken place between them in the region of the 

 Allegheny Mountains (see above the quotation from Campbell, 1896, 

 p. 675). In their course across the Piedmont Plateau these streams 

 are at present generally well separated, but farther to the east, where 

 they enter the region of the Coastal Plain, they reach a physiograph- 

 ical section of a character which permits frequent interchange of the 

 waters. In addition, we know that the Coastal Plain extended, at 

 certain times, farther seaward, and that the present Delaware and 

 Chesapeake Bays and also the estuaries of the other Atlantic streams 

 represent drowned river valleys, so that probably in the past this 

 interchange of the waters took place on a larger scale (see LeConte, 

 1891 ; Powell, 1896, p. yT,; Spencer, 1903; Davis, 1907, p. 717). 



Thus the Atlantic streams were not always isolated from each 

 other, and in the past, as well as in the present, an intercommunica- 

 tion of their waters was possible, chiefly on the Coastal Plain, which, 

 of course, also must have permitted an exchange of the faunas. 

 The importance of this will be understood below. 



