126 | ALLUVIUM. 
transition; others employ it to denote all aque- 
ous deposits of sufficiently recent formation to 
contain fossils of only the existing species of ani- 
mals and vegetables, and so make it include sev- 
eral series of the newer geognostic formations ; 
others employ it to denote all loose earthy strata, 
whether soil or subsoil, superimposed on indu- 
rated rock, and so make it include diluvium and 
every kind of detritus and mineral debris; others 
employ it to denote all aqueous deposits of a 
merely mineral kind, or deposits altogether or 
nearly free from animal and vegetable remains, 
and so make it exclude the vast aggregate ex- 
tent of the most recently formed strata which 
abound in shells, shrubs, grasses, and other or- 
ganic remains; and others employ it to denote 
all aqueous deposits of a merely mineral kind 
| constituting dry land, and so make it exclude 
the vast aggregate extent of quite new forma- 
tions in lakes, estuaries, bays, and gulfs, and 
| along the margin of seas and oceans. The two 
| last of these senses appear to us too contracted, 
and all the others too extended. The most an- 
cient aqueous deposits, altered by the action of 
fire, are properly metamorphic rocks; the next 
most ancient aqueous deposits, slightly if at all 
affected by the subsequent agency of fire, are 
transition or silurian rocks; the next great series 
of aqueous deposits, including very numerous 
members, and ranging from the old red sand- 
stone upward to the summit of the chalk forma- 
tions, are secondary rocks; the next series of 
aqueous deposits, consisting principally of the 
sands and clays nearest to the chalk formations, 
and of hard white sandstones and the newest 
limestones, are tertiary rocks; the loose aqueous 
deposits made by vast floods, particularly by the 
general deluge, and spread out in great expanses 
over hill and dale, and constituting a deep sub- 
soil in regions and districts where hard rocks 
cannot easily be found, are diluvium; and the 
loose deposits, effected by the attrition of the 
weather upon hard rocks, and constituting thin 
soils on the sides of rocky mountains and in 
other situations where the hard rocks of which 
they were formed occur near the surface, are 
detritus when lying on the very spots where they 
were disintegrated, and debris when washed only 
to such a distance as to retain their mineral char- 
acter unchanged. Now when all these various 
deposits have separate, distinguishing, and ex- 
plicit names, they cannot be made to share the 
additional designation of alluvium without the 
utmost violence to logic and the greatest confu- 
sion to nomenclature. On the other hand, to 
exclude from the name of alluvium the newest 
formations containing organic remains and the 
newest formations still submerged by lake and 
sea, is both to deprive the word itself of almost 
all possible meaning or application, and to assign 
to these formations a place or character hitherto 
unknown to science and unprovided with a name. 
We therefore understand alluvium to be all the 
newest aqueous deposits, made by river, lake, and 
sea, whether constituting actual land, or existing 
under water in a state of fitness to become soil. 
The existing inequalities on the surface of the 
earth, and particularly the origin, the form, and 
the distribution of valleys, have been the subject 
of much controversy and very conflicting opinions 
among geologists. Yet after all allowance has 
been made for volcanic action, or the formation 
of islands, hills, and mountains, by the sudden 
eruption of concentrated, local, subterranean 
fire,—for plutonic action, or the upheaving of 
mountain ranges, and the diversified elevating of 
broad tracts of country, by the power of diffused 
subterranean fire,—for disruptive action, in the 
crash of the avalanche, the fall of the landslip, 
and the devastations of the earthquake,—and for 
diluvial action, in the breaking up of continents, 
the dispersion of islands, the throwing down of 
hills, the filling up of hollows, and the general 
physical revolution of the world by the general | 
deluge,—all classes of geologists admit that a 
very large amount of the existing contour of the | 
earth, and especially those features and linea- 
ments of it which constitute the ramified basins 
of great rivers, must have been fashioned by the 
action of running water. Jn whatever condition 
the world was left by the general deluge, what- 
ever effects it retained of the previous great vol- 
canic and plutonic agencies, and whatever results 
it exhibited of the universal catastrophe which 
had just transpired, it cannot be imagined to 
have possessed the flowing outlines of valley and | 
the nice adjustments of river-course which now 
characterize it, but must be figured to the mind | 
as abounding in asperities, rugosities, spreading | 
tableaus, and sharply angular masses, which only | 
the erosions of the atmosphere, and the action of 
running water, could reduce to the existing con- 
dition of beautifully curved surface, and con- 
veniently intricate division. “ The first rains 
that fell, and the first springs which burst forth, 
would necessarily collect in the lowest levels, and 
thus the direction of the great trunk of a river | 
would be determined; and it might also happen 
that other clefts—depressions at a higher level— 
would communicate with this main channel. But 
that every such great depression would have a 
direct communication with the sea, and that 
such a combination of subordinate valleys as 
compose a river system, could have been formed 
by the breaking up of the earth’s crust, either 
by elevation or subsidence, can hardly, we think, | 
be maintained by any one. A river-course or 
system may be not inaptly compared to a picture 
of a great tree, whose branches gradually dimin- 
ish in size, but increase in number, as they re- 
cede from the stem. The great trunk of the 
river is divided into many branches, which spring 
from it at various distances from one another; 
and these again are subdivided into an infinity 
of smaller ramifications, each diminishing in size 
$$ ws 
as it increases in distance from the main trunk,— | 
