444 MEMOIRS OF THE CARNEGIE MUSEUM 



Mahoning Creek, (/rooked Creek, Two Tick, and Yellow ( 'reeks). Crossing over the 

 divide in this region into the drainage of the West Branch of the Susfjuehanna, no 

 trace of this species is found. I hunted for it in vain in Sinnamahoning Creek in 

 Cameron County, in the West Branch and its tributaries in ('learfield, Cambria, 

 and Indiana ( ounty (near ('berry Tree), and in ('learfield ( 'reekin ( "ambria County. 



In this whole region (headwaters of the West Branch) stream-piracy has taken- 

 place on a large scale, the whole basin of this river having been taken away from 

 the original Alleghany drainage. But (J. obscuras has not been taken over. Ac- 

 cording to Davis (1889, p. 248, see also above, p. 430) this stream-piracy fell largely 

 into Pretertiary times, and although we are to assume that it continued during sub- 

 sequent times (p. 430), it must have been rather slow, and insignificant, chiefly so in 

 Glacial and Postglacial times, which alone are to be considered in the case of C. oh- 

 scurus. Although this species was present in the Alleghany River drainage, it did 

 not go up into the headwaters, remaining away from the actual divide for a distance 

 of about ten to twenty miles. Under these circumstances, as stream-piracy was only 

 going on at the headwaters, no good opportunity was offered foi- this species to cross 

 the divide. 



In Cambria (bounty the continental divide bends to the east, and is transferred 

 to the main chain of the Alleghanies (Alleghany Front) ; but the eastern boundary 

 of (J. ohscvA-'US does not follow it. Here it is the (^'hestnut Ridge which constitutes 

 the boundary, beginning in southern Indiana County, and continuing through 

 Westmoreland and Fayette ( 'ounties to the southern state-line. Generally C. 

 obacurus does not pass beyond this ridge into the higher parts of the Alleghany 

 Plateau, Ijut theie are two exceptions. It is found in the Loyalhanna River in the 

 Ligonier valley, and in Indian Creek, and in this region it is not the Chestnut 

 Ridge, but the Laurel Hill Ridge which forms the eastern boundary. In the Cone- 

 maugh River and the Youghiogheny, this species has not been able to pass up- 

 stream beyond the (Jhestnut Ridge, since both rivers become very rough above this 

 point, and this roughness apparently existed also at the end of the Tertiary Period, 

 when the rivers descended, through the Chestnut Ridge, from the elevated Old Ter- 

 tiary peneplain to the late Tertiary base-level, at which they were then flowing.'" 



*' Acoordinu; to Campbell (1903, p. 292) the peneplain of southwestern Pennsylvania, elevation 1200 to 1300 feet, 

 is identical with the Old Tertiary Harrisburg peneplain ; and according to White (1896, p. 377), the Old Monongahela 

 ( with the Youghiogheny) of Late Tertiary age was about at base-level. Stevenson ( 1878, p. 259) has called attention 

 to an old terrace of the Youghiogheny at Connellsville, which apparently corresponds to the late Tertiary base-level, 

 200 feet above the present level (894 feet) at about 1100 feet above the sea. At Confluence it is 1820 feet high, thus 

 giving to the river betweeu Confluence and Connellsville a fall of about 700 feet at the end of the Tertiary At present 

 the fall of the river is only 432 feet between the points named. Although the identity of the old terraces is not demon- 

 strated, the difference of elevation is so great that a considerable fall of the Tertiary river is beyoud doubt, and thus at 

 that time a barrier to the upstream dispersal of C. obscuras must have existed here. 



