444 MEMOIRS OF THE CARNEGIE MUSEUM 
Mahoning Creek, Crooked Creek, Two Lick, and Yellow Creeks). Crossing over the 
divide in this region into the drainage of the West Branch of the Susquehanna, 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 Clearfield, Cambria, 
and Indiana County (near Cherry Tree), and in Clearfield Creek in Cambria 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 C. obscurus has not been taken over. Ac- 
cording to Davis (1889, p. 248, see also above, p. 450) 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 ease of C. ob- 
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 for this species to cross 
the divide. 
In Cambria County the continental divide bends to the east, and is transferred 
to the main chain of the Alleghanies (Alleghany Front); but the eastern boundary 
of C. obscwrus does not follow it. Here it is the Chestnut Ridge which constitutes 
the boundary, beginning in southern Indiana County, and continuing through 
Westmoreland and Fayette Counties to the southern state-line. Generally C. 
obscurus does not pass beyond this ridge into the higher parts of the Alleghany 
Plateau, but there 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 Chestnut 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.” 
58 According 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 between 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 beyond doubt, and thus at 
that time a barrier to the upstream dispersal of C. obscurus must have existed here. 
