TRENTON AND HUDSON PERIODS. 223 



It is possible that this barrier, or reef, extended along the present 

 position of the Green Mountains ; but to the south, in Pennsyl- 

 vania and Virginia, it must have lain to the eastward of the Appala- 

 chian region, since the Trenton rocks in part constitute that por- 

 tion of the chain. The depth of water may have been small, if we 

 judge from what is necessary for the formation of such limestones 

 at the present day. Any subsidence in progress during its forma- 

 tion must have been exceedingly slow, — less than half an inch a 

 year; for otherwise the animals forming the limestones out of 

 their shells would have been destroyed by being sunk below the 

 depth at which they could live. There was, beyond doubt, such a 

 slowly-progressing subsidence ; and in the Appalachians it ex- 

 ceeded in the end near 6000 feet (the thickness of the Trenton 

 limestones), while over the interior it was, in general, not far from 

 300 feet. 



The great St. Lawrence bay about Ottawa still existed, as in the 

 Potsdam period, though probably somewhat contracted in size. 

 (Map, fig. 205.) Here the subsidence must have been nearly 1000 feet. 



The thin layers of shale in the Trenton limestone are no more 

 than would have naturally been formed by streams flowing from 

 the northern Azoic. 



But with the opening of the Hudson period shales were formed 

 in Canada and from the Hudson Eivef westward along the northern 

 border of the United States. They have their greatest thickness 

 to the eastward, where they extend from Canada and New York 

 southwest, along the Appalachian region ; but over the middle of 

 the interior basin, limestones (though often somewhat mixed with 

 shales) were still in progress. It is evident that some great change 

 had taken place. Probably the subsidence along the northern 

 border of the United States (south of the Canada Azoic), and also 

 along the Appalachian region, had been increased in rapidity, until 

 the forming limestone reefs were submerged and the animals about 

 them destroyed, — thus putting an end to the further increase of 

 the calcareous beds, and leaving them as the foundation for future 

 deposits. (A subsidence of the coral islands of the Pacific of 300 

 feet would wholly extinguish the life of the reef-forming corals.) 

 The eastern continental barrier may also have been partly sub- 

 merged, so as to admit the tidal and great oceanic currents, while 

 other parts of it were just washed by the waves which swept from 

 it and bore westward the finer detritus for portions of the Hudson 

 River shales. Had the waves of the ocean entered in full force, 

 beds of pebbles would have been more common among the de- 

 posits. Wherever the layers are somewhat arenaceous and full of 



