Devonian Era in Ohio Basin. — Claypole. 339 
in 1881, gives yet greater range to some members of the Mar- 
cellus fauna. He concludes : 
"1. That the same fauna, which was well defined in the Marcellus 
shale :ecurs with very slight modification in the Genessee shale and the 
Ithaca shale of New York. 
2. That this recurrent fauna of New York is the same fauna which 
characterizes the Black shale of Ohio, Kentucky, Indiana, etc., and 
other regions in the interior of the continent. 
3. That the fauna was a single continuous fauna and during the 
e?.rly and middle Devonian age its centre was located to the southward 
and westward of New York." 
Regarding the rirst of professor Williams's conclusions there 
is no ground for disagreement. The facts quoted by him and 
others, which have come to light since 1881 afford ample proof 
of its truth. But the second does not appear to be equally well 
founded. The molluscan fauna of the Black shale of Ohio, 
etc., is far too scanty to justify the deduction. 
Another illustration of this recurrence of species with the 
recurrence of congenial conditions may be quoted from Penn- 
sylvania. The genus Rensseheria, with all its forms and sub- 
divisions, is especially abundant in the Oriskanv sandstone, 
but disappears with the replacement of this material with 
shale in the Marcellus and Hamilton strata. In middle 
Pennsylvania, however, the medial mass of the Hamilton 
is a coarse, -thick sandstone, and with this reappears the 
genus Rensselseria (Newberria). This raises the question, 
as in the cases quoted by professor Williams, "Where had the 
genus resided during the interval when the conditions were 
uncongenial?" It would seem most reasonable to think that 
the place of its retreat was at no very great distance and, ap- 
parently, it must have been near the shore line of that day, as 
only there could it find the kind of bottom in which it seems 
to have delighted. The solution of this problem is possibly 
more dependent upon geographical extension and migration 
than upon succession in time. 
VII. ABUNDANCE OF PETROLEUM AND SCARC- 
ITY OF FOSSILS IN THE SHALE. 
The abundance of petroleum and the great scarcity of fos- 
sils in the Black shale are facts that point in two directly op- 
posite directions. The former is indisputable evidence of the 
alumdancc of organic matter, and therefore, of organisms liv- 
