in the Geological History of North America. 343 



again, and varying in its accumulations from sandstones to shales, 

 pebble beds or limestones, with the depth and the currents ; and 

 then again above the tides, although destruction to all the life 

 of the ocean was in the movement ; and, perchance, lying in the 

 open air for an era, to receive the mists and rains and sunshine, 

 and become luxuriant through new creations tfith broader 

 prairies than now cover the West. Alternations like these were 

 again and again repeated, as geology has shown. 

 / Through these means, the continent, which was begun at the 

 far North, a region then tropical but afterwards to become in- 

 hospitable, gradually expanded southward, area after area as time 

 moved on being added to the dry land^X 



First, as the facts show, the Silurian deposits of Canada and 

 the North, adjoining the Azoic, were left above the sea, for these 

 rocks there are not overlaid by later beds ; and, therefore, were 

 not the sea-bottom of later seas. Next, the adjacent Devonian 

 were added to the main land as far south as Southern New York 

 and around by the west ; for, as the New York geologists have 

 shown, the carboniferous beds which come next do not reach 

 into that State. By the time of the Jurassic period, the continent 

 had expanded much farther to the southward, for the carbonif- 

 erous rocks over the land were out of water, their beds having 

 already been folded up and elevated in the Appalachians. The 

 red sandstone of the Connecticut Valley and of the Atlantic 

 States from New York to Alabama leave little doubt as to the 

 water line of that era. In the Cretaceous period the continent 

 had farther expanded along the Atlantic ; but in the Mississippi 

 Valley the Mexican Gulf still extended north even to the head 

 waters of the Missouri. Next, as the Tertiary opened, the con- 

 tinent had yet more widely enlarged its bounds, south and south- 

 east ; and if the waters of the Mexican Grulf for a while claimed 

 a place over some part of the Nebraska plains, as late observa- 

 tions suggest, by the close of the period the continent in this 

 direction had nearly reached its full maturity. These steps of 

 progress are indelibly marked in the position, and obvious sea- 

 coast, off-shore or estuary origin of the Jurassic, Cretaceous and 

 Tertiary beds of the country. 



Passing towards the Pacific, we find evidence in the carbonif- 

 erous limestone that the Eocky Mountains were mostly under 

 shallow water as the Carboniferous age opened, the mountains 

 themselves unborn. Later in the Cretaceous and Tertiary periods, 

 as the rocks towards the coast testify, the continent had extended 

 far to the southwest, and was nearly complete in that direction, 

 as well as to the south and southeast. 



Thus the enlargement went on to the southward, each period 

 making some addition to the main land, as each year gives a 

 layer of wood to the tree. Not that this addition was free from 



