2l6 MOUNDS NEAR THE ESTUARY OF THE THAMES.
folds, some of them even inverted. As these crumble down under
the action of the weather they develop into small mounds having
much the same form as those at Sheppey. The conditions requisite
for the formation of ridges in this way must have been present again
and again in both the Sheppey area and that at Sea Salter.
I would suggest a second possible explanation. All along the
foot of the slope extending Thameswards from the North Downs,
the strata are as a rule more or less saturated with water percolating
downwards along the planes of bedding, in many cases representing
the underground drainage derived from the whole outcrop. Near
the foot of the slope, the upper limit of underground saturation
cuts the surface at or near the sea level, and springs are thrown out
as a consequence. Hundreds of such springs occur along the zone
referred to, all the way along the foot of the northward slope border-
ing on the Thames, and the Swale, between Chatham and Whit-
stable, and it is from these sources that so many of the smaller
seaward-flowing streams take their origin, as anyone may see by
examining a good geological map. Now it must often have happened
that some of these springs burst forth over an area where fluvio-
marine alluvium was being laid down. The natural result must have
been a conflict between the effort of the spring to relieve the pres-
sure of water behind it, and that of the alluvial mud to seal up the
orifice. Under these circumstances, a kind of mud volcano must
have resulted again and again as the springs changed their position
and worked their way up through the alluvial mud to the surface at
different places. Many of the mounds present exactly such features
as would result from the deposition of cones of marsh mud brought
up by springs at the time when the marsh land was flooded by alluvial
matter.
There is yet another, and, as I think, a better explanation still—
better because it will apply to all the instances that have yet come
under my own notice. That is, that these mounds are simply "mud
lumps" which have originated in the same manner as those des-
cribed by Messrs. Humphreys and Abbot in their Report on the Mis-
sissippi.3 Sir Charles Lyell ("Principles," vol., i., ch. xix.,pp. 443-450,
gives an abstract of these observations. He notes that the mud lumps
referred to are always situated off the mouth of the rivers, that they
all consist of homogeneous tenacious mud. " This mud is chiefly
pushed up bodily, but some of it consists of matter brought up by a
3 My attention was first called to this point by my colleague, Mr. Topley.