21 DIFFERENT ORIGIN OF SECONDARY [Ch. III. 



to subaqueous deposits formed during the period when the sea 

 prevailed, as contrasted with those that might belong to the 

 subsequent epoch when the land should predominate. First, 

 we may suppose a vast submarine region, such as the bed of 

 the^vvestern Atlantic, to receive for ages the turbid waters of 

 several great rivers, like the Amazon, Orinoco, or Mississippi, 

 each draining a considerable continent. The sediment thus 

 introduced might be characterized by a peculiar colour and 

 composition, and the same homogeneous mixture might be 

 spread out over an immense area by the action of a powerful 

 current, like the Gulf-stream. First one submarine basin, and 

 then another, might be filled, or rendered shallow, by the influx 

 of transported matter, the same species of animals and plants 

 still continuing to inhabit the sea, so that the organic, as well 

 as the mineral characters, might be constant throughout the 

 whole series of deposits. 



In another part of the same ocean, let us suppose masses of 

 coralline and shelly limestone to grow, like those of the Pacific, 

 simultaneously over a space several thousand miles in length, 

 and thirty or forty degrees of latitude in breadth, while vol- 

 canic eruptions give rise, at different intervals, to igneous 

 rocks, having a common subaqueous character in different 

 parts of the vast area. 



It is evident that, during such a state of a certain quarter 

 of the globe, beds of limestone and other rocks might be 

 formed, and retain a common character over spaces equal to a 

 large portion of Europe. 



State of the Surface when the Tertiary Groups were formed. 



But when the area under consideration began to be con- 

 verted into land, a very different condition of things would 

 succeed. A series of subterranean movements might first give 

 rise to small rocks and isles, and then, by subsequent eleva- 

 tions,, to larger islands, by the junction of the former. These 

 lands would consist partly of the mineral masses before de- 

 scribed, whether coralline, sedimentary, or volcanic, and partly 



