468 MEMOIRS OF THE CARNEGIE MUSEUM 
its former presence. This is apparently due to the large amount of pollution in 
these streams, chiefly by water from mines. The pollution of the Alleghany River 
near Pittsburgh, although bad enough from a sanitary standpoint, and due in the 
first instance to sewage (Leighton, 1903, p. 122) does not affect the crawfishes, for 
they are very abundant here, and the Ohio below Pittsburgh is rich in crawfishes. 
But there are many smaller streams contaminated by the waste of coal-mines.” 
Such streams are recognized at a glance by the precipitate of reddish and yellowish 
sulphate of iron upon their bottoms, and are invariably without life. This is most 
evident in the Monongahela drainage of southwestern Pennsylvania (Washington, 
Fayette, and southern Allegheny Counties), and also in many smaller streams in 
Butler, Westmoreland, Indiana, and Jefferson Counties, where coal-mines are abun- 
dant. The worst conditions prevail in certain tributaries of the Monongahela, 
in the Monongahela itself, in the Loyalhanna below Latrobe and the Kiskimin- 
etas, and in Red Bank and Sandy Lick Creeks. The Clarion River is also with- 
out crawfishes in Jefferson and Elk Counties, but this is due chiefly to pollution 
by sewage from wood-pulp mills and tanneries (see above, p. 443). In all these 
cases it is evident that C. obscuwrus once existed here, since remnants of it are left at 
many places in some of the clearer and not polluted side streams. Since this pol- 
lution of the streams by coal-mines is bound to increase, C. obscwrus certainly will 
disappear from other streams. As we have seen above it was on the point of dying 
out in Sandy Lick Creek at Du Bois in 1905 (p. 443, footnote 58). Another case has 
been observed in Fern-Hollow Run, Pittsburgh. In the fall of 1903 I found a 
small number of specimens of this species left over in some pools once connected 
with the run; a sewer had recently been built here, discharging its polluted water 
into the run. In subsequent years this species was not again seen, and has entirely 
disappeared, as also from Nine-Mile Run, which receives sewage from Wilkinsburg 
and Edgewood. 
It should be added that C. bartoni also is frequently influenced by the contamina- 
tion of streams, but seems rather more resistant than C. obscwrus. In two cases this was 
evident, namely, in Mahoning Creek at Punxsutawney, and in Slippery Rock Creek 
at Branchton. In both cases the streams were only slightly polluted by mine-water, 
and contained a certain number of specimens of C. bartoni, while C. obscwrus was 
absent. The latter existed at Punxsutawney in a pond connected with the stream, 
and at Branchton in a smaller clear tributary, and consequently must have once 
been present in the two creeks. 
A stream or river polluted in a certain part becomes relatively clear and pure 
°7 As to the chemical processes going on in the so-called ‘‘ sulphur water,’’ see Leighton, 1904, p. 24. 
