THE DEVONIAN PERIOD. 427 



conditions changed so as to give origin to deposits of mud where the 

 Onondaga limestone had been accumulating. These mud beds and 

 their equivalents, now consolidated, constitute the Marcellus and 

 Hamilton formations of New York (see Fig. 195). West of Ohio and 

 east of the Mississippi the equivalents of the two formations are com- 

 monly grouped together under the name Hamilton, or given local 

 names (see Appendix, Vol. III). Shale is the most common rock at 

 the east, but with it is associated not a little limestone, and in the 

 west, limestone predominated. Southward, in Kentucky and Tennessee, 

 the equivalent formations, where present, 1 consist largely of black 

 shale and slate. 



These changes in sedimentation appear to have been accompanied 

 by changes in geography (compare Figs. 195 and 193). During the 

 early part of the Marcellus epoch, the connection between the Atlantic 

 and the interior by way of the Chesapeake may have been re-established. 2 

 At about the same time, too, considerable areas in the southern and 

 in the northwestern parts of the Mississippi basin appear to have been 

 submerged. In the Mississippi basin, the Hamilton formation prob- 

 ably overlaps its predecessor, resting on Silurian and perhaps even 

 on Ordovician beds. The same relations hold in the southern Appa- 

 lachians, where Devonian beds, probably the equivalents of the Mar- 

 cellus and Hamilton, overlie Silurian and Ordovician unconformably. 

 The submergence of the land in the south appears to have been suffi- 

 cient to cause the sea to overspread areas which had been land since 

 the close of the Ordovician (southern Appalachians and areas farther 

 west) and perhaps to open up a connection between the interior sea 

 and the Gulf of Mexico, allowing shallow-water species of animals to 

 migrate into the Mississippi basin from the south, perhaps from South 

 America (see Hamilton fauna, p. 460). The Cincinnati arch and the 

 corresponding Nashville dome may have been land (islands or a penin- 

 sula, Fig. 195) throughout the Hamilton epoch, though this cannot 

 be affirmed. If the Hamilton formation once overspread them, it 

 has been removed by erosion. The map, Fig. 195, shows the general 

 relations of the series, and at the same time suggests the geography 

 of the epoch. 



1 Hayes and Ulrich, Columbia (Tenn.) folio, U. S. Geol. Surv. This folio gives 

 a late correlation table for the Ohio-Kentucky-Tennessee region. 



2 Ulrich and Schuchert, Bull 52 (Paleontology 6) New York State Museum, Report 

 of the State Paleontologist, 1902, p. 654. 



