MOUNDS NEAR THE ESTUARY OF THE THAMES. 217
spring to the top of a dome-shaped mass, and thrown down on the
surface on all sides over which the muddy water pours. So much
carburetted hydrogen .... is emitted, together with salt or brackish
water, that Col. Sidell suggests that they deserve the name of gas
vents rather than of springs." (Lyell, op. cit., p. 441.) Sir Charles
goes on to say (p. 449), referring to the alluvial matter deposited
by the Mississippi, that it "forms annually a mass of no less than
one mile square, having a thickness of twenty-seven feet. It con-
sists of mud, coarse sand, and gravel, which the river lets fall some-
what abruptly when it comes into contact with the still salt water of
the Gulf. A cubic mass of such enormous volume and weight thrown
down on a foundation of yielding mud .... may well be conceived
to exert a downward pressure capable of displacing, squeezing, and
forcing up some parts of the adjoining bottom of the Gulf, soon to
give rise to new shoals and islands."
The marked parallel between the cases cited and those here
specially under notice will, I think, strike everyone who looks into
the matter. Both in America and in all the occurrences yet made
known in England, these problematical mounds occur just at the
point where a river bringing down fresh water charged with sediment
and with much organic matter is met by the salt water of the ocean.
A rapid throwing down of mud takes place, concurrently with the
destruction and entombment of organic matter, and with the con-
sequent evolution of gases resulting from its decomposition. Gas
vents and springs of water break through the alluvial mud as fast as
this is formed, and cone after cone of mud is piled up here and there
as the vents change their position. Subsequently, the deposition of
alluvial matter ceases, and then, with the change of conditions, the
piling up of the cones ceases also. After a time the area is sub-
merged only during floods, and the alluvium thrown down dries in
quite horizontal sheets abutting against the slopes of the mounds.
In the case of many of the English mounds, it seems more than
likely that early man took advantage of them for his own purposes,
sometimes converting them into grave mounds, as he shaped tumuli
out of the eskers of Northern England; or he may have turned
them into homesteads, or have used up their clay on the spot for his
rude kinds of pottery, or have turned them to any other use suggested
by his special need at the time.
What I suggest is, that the Kentish mounds are mainly natural in
origin, that they may be due to a variety of causes (chiefly, I think,