THE DEVONIAN PERIOD. 425 



over the area where the clastic beds had been gathering, though locally 

 clastic sedimentation may have continued through the epoch of the 

 Onondaga limestone. 1 The general distribution of the formation 

 and the general geography of the epoch are suggested by Fig. 193. 

 A comparison of this map with Fig. 191 will give some idea of the 

 changes which are thought to have taken place during this epoch. 2 



While the Onondaga limestone does not appear at the surface over 

 extensive areas in the eastern part of the United States (Fig. 193), 

 the area which it actually covers is much greater, since it is largely 

 buried by younger formations. North of the United States, the forma- 

 tion occurs on the west side of James Bay (south of Hudson Bay). 

 As will be indicated in connection with the account of the Devonian 

 faunas, the new element in the Onondaga fauna of the interior may 

 have come into the United States from the north. If so, the Onondaga 

 of the Hudson Bay region was once connected with that of the inte- 

 rior of the United States. 3 The distribution of such formations as 

 this and the Niagara (p. 377), convey some idea of the extent to which 

 erosion has removed formations from regions which they once covered. 

 The new types of life which appeared in the Onondaga, that is, the 

 forms by which it is distinguished from earlier formations of the interior, 

 seem to have reached Ontario and Michigan, perhaps from the north, 

 at about the same time that the Oriskany types, entering the interior 

 from the Atlantic, reached the same place. Such relations show the 

 importance of paleontological studies in determining the geography 

 of the past. 



The constitution of the main body of the Onondaga formation 

 shows that the water of the interior sea was relatively clear, and this 

 in turn suggests that the surrounding lands were not sufficiently high 

 to allow of rapid subaerial erosion. The limestone is often rich in 

 corals, and the coral-reef structure is sometimes as perfectly shown 

 as in the reef rocks of modern times. This is true, for example, at the 

 rapids of the Ohio near Louisville. 



1 The Newfoundland grit of New Jersey may be equivalent to the Onondaga lime- 

 stone farther west: Weller, Report on the Paleozoic Paleontology of New Jersey, 

 State Geol. Surv. of New Jersey, 1903. 



2 For an account of the Devonian in the Ohio basin, see Claypole, Am. Geol. , Vol 

 32, 1903, pp. 15-41 and 79-105. 



3 Weller, Jour, of Geol., Vol. X, pp. 423-32. 



