412 On American Geological History. 



whole process have been utterly frustrated by hot water, or by 

 mere changes of level in the earth's crust, and creation would 

 have been at the mercy of dead forces. The surface would have 

 required again and again the sowing of monads, and there would 

 have been a total faihire of crops after all ; for these extermina- 

 tions continue to occur through all geological time into the 

 Mammalian Age. 



ir. Airain : I have observed that the continent of North 

 America has never been the deep ocean's bed, but a region of 

 comparatively shallow seas, and at times emerging land ; and 

 was marked out in its great outlines even in the earliest Silurian. 

 The same view is urged by De Verneuil, and appears now to bo 

 the prevailing opinion among American geologists. The deptli 

 at times may have been measured by the thousand feet, but not 

 by miles. 



III. Durinof the first half of the lower Silurian era, the whole 

 east and west were alike in being covered w^ith the sea. In the 

 first on Potsdam Period, the continent was just beneath or at the 

 surface. In the next or Trenton Period, the depth was greater, 

 giving purer "waters for abundant marine life. Afterwards, the 

 east and west were in general widely diverse in their forma- 

 tions ; limestones, as Mr. Hall and the Professors Rogers have 

 remarked, were generally in progress over the west, that is, the 

 region, now the great Mississippi Valley, beyond the Appala- 

 chians, while sandstones and shales were as generally forming from 

 northeastern New York south and southwest through Virginia. 

 The former therefore, has been regarded as an area of deeper 

 waters, the latter as, in general, shallow, when not actually 

 emerged. In fact, the region towards the Atlantic border, after- 

 wards raised into the Appalachians, was already, even before the 

 Lower Silurian era closed, the higher part of the land : it Jay as 

 a great reef or sand-bank, parlly hemming in a vast continental 

 lagoon, where corals, encrinites and mollusks grew in profusion, 

 thus separating more or less perfectly the already existing Atlantic 

 from the interior waters, 



IV. The oscillations or changes of level over the continent, 

 through the Upper Silurian and Devonian, had some reference 

 to this border region of the continent : the formations approach 

 or recede from it, and sometimes pass it, according to the limits 

 of the oscillations eastward or westward. Along the course of 

 the border itself there were deep subsidences in slow progress, 



