THE DEVONIAN PERIOD 563 



near Louisville. The formation is not a thick one, rarely more than 

 100 to 200 feet. 



The equivalent of the Ulsterian series occurs east of the Appala- 

 chians in northern New England and Canada, having a distribution 

 similar to that of the Helderberg. The formation occurs also on the 

 west side of James Bay (south end of Hudson Bay), and the rem- 

 nants here may have been connected formerly with one another, 

 and with the equivalent formation of the interior of the United 

 States. 1 The distribution of such formations as this and the Niagara 

 (p. 539) give some idea of the extent to which erosion has removed 

 formations from regions which they once covered. 



Following the Onondagan epoch of clear seas, conditions changed 

 so as to give origin to deposits of mud where limestone had been 

 accumulating. These mud beds and their equivalents, now con- 

 solidated, constitute the Marcellus and Hamilton formations of New 

 York (p. 559). In the interior, the equivalents of the two forma- 

 tions are commonly grouped together under the name Hamilton, 

 or given local names. In the east, shale is the most common rock, 

 but in the west, there is a good deal of limestone. 



Considerable areas in the southern and northwestern parts of 

 the Mississippi basin which had been land in the earlier part of the 

 period appear to have been submerged in this epoch, for the Ham- 

 ilton formation probably overlaps its predecessor in these directions, 

 resting on Silurian beds. The spread of the sea at this time, begin- 

 ning perhaps a little earlier, appears to have submerged areas in 

 the south (southern Appalachians and areas farther west) which 

 had been land since the close of the Ordovician, and perhaps 

 opened up 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. The Cincinnati arch may 

 have been land throughout the Hamilton epoch, though this can- 

 not be affirmed. If the Hamilton formation once overspread this 

 arch, it has been removed. 



The conditions for the origin of the Hamilton shales would seem 

 to be met if the surrounding lands (Appalachia and lands north of 

 the interior sea), after standing low while the Onondaga limestone 



1 Weller, Jour. Geol., Vol. X, pp. 423-432. 



