PHYSICAL AND FAUNAL EVOLUTION 247 
of Devonic affinities. This is further shown by the occurrence of 
Panenka and Hercynella in these beds. The highest division (Lucas) 
is characterized by gastropods, most nearly related to late Siluric 
types of northern Europe. 
The Amherstburg beds of the Upper Monroan appear to be the 
chronologic equivalent of the Cobleskill of eastern New York, several 
characteristic species being common to both. It represents the 
junction of an eastern and a western sea, and a commingling of the 
fauna of both. The typical Upper Monroan coral and brachiopod 
fauna seems to have invaded Michigan from the northwest, a 
somewhat similar fauna appearing near the headwaters of the 
Saskatchewan. In Pennsylvania the Lewistown limestone appears 
to represent this horizon. 
The Sylvania sandstone has all the characteristics of a wind- 
drifted sand. Its cross-bedding is of the aeolian type, its grains well 
rounded, pitted, grooved, and of uniform size; there is a total absence 
of impurities, and all the characteristics compare favorably with 
those of the sands of the Lybian desert of today. It indicates a period 
of land condition between the retreat of the Atlantic embayment 
(Lower Monroan) and the Pacific invasion of Upper Monroan time. 
G. THE LOWER DEVONIC 
The Lower Devonic comprises the Helderbergian and the Oris- 
kanian of Clarke and Schuchert. The Helderbergian includes the 
Coeymans, New Scotland, Becraft, and Port Ewen. The latter is 
transitional to the Oriskany, and Chadwick proposes to unite it with 
that formation.' The Coeymans is the direct depositional successor 
of the Manlius, there being frequently a transitional zone between 
them, with a commingling of the fossils.. The former extent of the 
Coeymans can be estimated from its occurrence at Syracuse and the 
uniform character which it maintains in that region. This indicates 
that the western shore of the Helderberg sea was west of Syracuse 
and perhaps in the region of Buffalo. The eastern and northern limit 
of the formation is indicated by its mergence into shore deposits in 
New Jersey, and the southward overlap of the later formations, 
t Science, N. S,, Vol. XXVIII, p. 347. 
