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 western 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 
inti'oduced 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 
