THE DEVONIAN PERIOD. 429 



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), low and protected while the Onondaga limestone was 

 making, were more elevated or less protected by vegetation, or sub- 

 jected to more concentrated or spasmodic precipitation. Under 

 the former conditions, the land formations would have been under- 

 going decay, but the products of the decay might not have been removed ; 

 under the latter, there would have been opportunity for the trans- 

 portation of the products of decay. Even during the general period 

 of shale formation, however, limestone (often shaly) was making in 

 many places. This was especially true in the Mississippi basin, so 

 that while the term "Hamilton shales" is in common use, "Hamilton 

 shales and limestones" would describe more accurately the Erian 

 series of this region. The Marcellus shale is locally more or less 

 bituminous, indicating the presence of abundant plant life in the seas 

 in which it was deposited. 



In the east, the Hamilton (including Marcellus) formation is very 

 thick, being 1500 to 5000 feet thick in Pennsylvania, where it is 

 mainly of fragment al origin. Its thickness over the interior, where it 

 contains more limestone, is but a fraction of this amount. 



Middle Devonian in the northwest. — A considerable area of Devo- 

 nian which has sometimes been called Hamilton is found in the basin 

 of the Mackenzie river and southward to Manitoba. 1 The great arm 

 of the sea in which the Devonian of this area accumulated appears 

 to have extended as far south as northern Missouri (Fig. 195). Whether 

 this arm of the sea antedated the Hamilton epoch is uncertain. Devo- 

 nian beds, some of which may be the equivalent of the Hamilton, 

 also outcrop along the main ridge of the Canadian Rockies. 



The fossils of this northwestern Devonian do not correspond with 

 those of the Hamilton formation of the east (Illinois to New York), 

 and if the beds of the two regions were contemporaneous, as they may 

 have been, they seem to have been deposited in waters which were 

 not connected. The union was probably prevented by a narrow belt 

 of land running south-southwest from Wisconsin to Missouri, some- 

 what as shown in Fig. 195. Till late in the Hamilton, this land seems 

 to have constituted a barrier between the eastern interior sea, and a 



1 For review of the Devonian of Canada, see Whiteaves, Am. Geol.,Vol. XXIV, 

 1899, pp. 210-40. 



