404 DEVONIAN PERIOD 



and (3) the lower Mississippi basin (Fig. 356). The Oriskany 

 formation, chiefly sandstone in the east, has a similar, but some- 

 what wider, distribution. It is best known in the northern Appa- 

 lachian region. From the vicinity of Cumberland, Md., where it has 

 a thickness of a few hundred feet, it thins to the northeast and south- 

 west, and loses its most distinctive faunal characteristics as it thins. 



The Middle Devonian. The Middle Devonian is more wide- 

 spread than the Lower and its most important formations are the 

 Onondaga and Hamilton. The Onondaga limestone is found from 

 New York to the Mississippi (Fig. 357), resting on Silurian beds 

 with little evidence of unconformity. The epicontinental sea in 

 which it was formed was relatively clear and shallow, as shown by 

 the composition of the rock and its fossils. In many places the 

 limestone is rich in coral, and locally the coral-reef structure is 

 shown perfectly. This is true, for example, at the rapids of the 

 Ohio near Louisville. The formation is rarely more than 100 to 200 

 feet. In northern New England and Canada, the equivalent of 

 this formation has a distribution similar to that of the Lower Devo- 

 nian. It occurs also on the west side of the south end of Hudson 

 Bay, and the beds here may have been connected formerly with 

 equivalent formations in the interior of the United States. 



Following the Onondagan epoch of clear seas, conditions changed 

 so as to give origin to deposits of mud in many places where lime- 

 stone had been forming. These mud beds, now consolidated, con- 

 stitute the Marcellus and Hamilton formations of New York (p. 402). 

 In the interior, where there is more limestone, the equivalents of 

 the two formations are commonly grouped together under the name 

 Hamilton, or given local names. 



Considerable areas in the southern and in the northwestern parts 

 of the Mississippi basin which had been land earlier, appear to have 

 been submerged at this time (Fig. 358), for the Hamilton formation 

 appears to overlap its predecessor in these directions, resting on the 

 Silurian. The spread of the sea at this time seems also to have 

 submerged areas in the southern Appalachians which had been land 

 since the close of the Ordovician. Connection may have been made 

 at this time between the interior sea and the Gulf of Mexico, allow- 

 ing shallow-water species of animals to migrate from the south into 

 the Mississippi basin. 



The conditions for the origin of the Hamilton shales would seem 

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



