UPPER SILURIAN. 257 



cylindrical fossils found in the island of Anticosti (p. 231) have 

 been referred to plants and named Beatricea. Similar fossils occur 

 also in Kentucky. x But the ambiguous character of the fossils, 

 their occurrence in limestone, and the non-occurrence of any land- 

 plants in the marls — originally marshes— of the Salina period, 

 make their vegetable nature very doubtful. It is not impossible 

 that leaves and stems of true land-plants may yet be found ; but 

 it is very remarkable that, if existing, the beds of shale and ar- 

 gillaceous sandstone should have been so extensively explored 

 without finding them. 



Conditions of the Continent. — The survey of the successive 

 formations of the Upper Silurian teaches that the geological 

 changes in progress were, like those of the earlier Silurian, of con- 

 tinental extent. The causes in action were not making a mere 

 edging to the continent, as in Tertiary times, but were building 

 up the very continent itself by wide-spread accumulations of lime- 

 stone, sands, and clays. 



Moreover, the continental seas were not the ocean's bed. In 

 many of the epochs, the ripple-marks and cracks from sun-drying 

 prove the shallowness of the water over great regions and a wide 

 expanse of exposed beaches and marshes in others. The corals 

 and the profusion of marine life in the limestones are also proofs 

 of shallow waters. No greater depth than 500 feet is indicated 

 by any of the species ; and as corals were probably limited then, 

 as now, to within twenty fathoms of the surface, — for they can 

 make solid limestones only where the waves can help them, — the 

 continental seas must have been to a considerable extent much 

 less deep. 



The continental areas still included little permanent dry land. 

 The continent had enlarge^, somewhat since the Azoic age ; but the 

 greater part of the United States was yet to be completed by the 

 deposition of the Devonian, Carboniferous, and later beds. 



Contrast between the Interior and the Appalachian regions. 

 — The shales and sandstones which prevail in the East from the 

 vicinity of the Azoic of New York southwest along the Appala- 

 chian region are mostly wanting in the West, where the Niagara 

 limestone is widely distributed. In some places on the north 

 there is a limestone of the Salina period. The West was there- 

 fore in certain parts still making limestones while the East inter- 

 posed between its limestones extensive clay and sand deposits. 

 The limestones of the West prove slight changes of level there ; the 

 argillaceous beds and sandstones of the East, great oscillations 

 over the Appalachian region ; and in the Niagara period they 



18 



