16 On the Physical Geology of the United States, Sc. 
Thus far we have considered the causes of the great equili- 
brating currents of the ocean; the physical laws regulating their 
circulation, and the influences of these currents on organic life, 
and on the deposition of the sedimentary strata. It remains to 
consider whence the materials of the sedimentary rocks have 
been derived, that have been transported. by currents, and depos- 
ited over so vast an area and of stich great thickness in the Uni- 
ted States. 
If the great currents of the ocean have flowed in times past as 
we have shown from physical causes they must be supposed to 
have flowed, the greatest proportion of transported earthy matter 
in the northern and eastern parts of the United States (except 
between the mountains and coast) must have been brought from 
the northward and spread to the south and southwest,—the gen- 
eral trend of transport according to the physical law that has 
been explained, tending to the southwest by means of the polar 
current. Other large quantities, together with tropical. plants 
and animals transported by the equatorial current in its north- 
wardly flow, would be spread over the areas occupied by the 
sedimentary rocks of the United States and British possessions, 
from the south to the north and northeast; and by the blending 
of the currents, and the deflections caused by this and by obsta- 
cles in particular parts, would be spread in various directions as 
we now find them. 
Of the materials swept from the south and east by the equa- 
torial current, we can have little direct evidence. This current 
sweeps, and has in times past swept over vast areas of the bed 
of the ocean, and along coasts and reefs of rocks, from which 
large quantities of detrital matter might, in the course of un- 
numbered ages, be supposed to have been swept away and trans- 
ported to distant parts. Of the capacity of such a current to 
transport floating plants in ancient times to form our coal deposits, 
and the various traces of vegetation so common in all our sedi- 
mentary rocks below the coal formation, we have only to look at 
the effects of the present Gulf Stream, one branch of which 
carries large quantities of drift timber from the coasts of South 
America, the Gulf of Mexico, and the shores of the Atlantic, 
and lodges them on the shores of Labrador, Greenland, Iceland, 
Spitzbergen, Norway, and the Scottish islands ;* and the other 
* Lyell’s Geology, Lond. edition, 1833, p. 251; Malte Brun’s Geography, ce 
I, p-112; Edinburgh Encyclopedia, 
