186 Notes on the Geological Position of the Human 
account of the nature and formation of alluvial deposits will 
probably form a useful preface to my other remarks. 
If we glance at the map of the Geological Survey, showing 
the valley of the Thames between London and Gravesend, we 
see that the Chalk and Tertiary rocks of Kent are separated 
from those of Essex by a broad band of alluvium or marsh¬ 
land, which occupies the ground between the bends of the 
river. And outside this tract of marsh-land, but still in the 
neighbourhood of the Thames, we see less continuous patches 
(of other colours) of gravel and brick-earth, wdiich rest 
indifferently on Chalk, Thanet Sand, Woolwich Beds, or 
London Clay, and whose nature, distribution, and fossil 
contents ah testify to their formation by the river at former 
periods of its existence. These last-named beds are all, 
however, to be found at various heights above the level of the 
marsh-land, though the old river-gravel also underlies the 
marsh alluvium. When a tract of land emerges from the 
sea, the rain falling upon it tends to form brooks and rivers 
where slight hollows exist. These hollows gradually become 
deeper and deeper, from the erosive action of the stream and 
its tributaries, and should a slow continuous upheaval of the 
land accompany the deepening of the river-valley by the 
stream, so as to preserve its angle of slope towards the sea, 
a river-valley as deep as that of the Grand Canon of the 
Colorado may in time be formed. On the other hand, should 
a subsidence of the area take place, the river, having its fall 
lessened, soon no longer tends to deepen its valley, but to 
meander in bold curves and to widen it, forming, as it does 
so, broad alluvial flats with the material brought down in 
suspension. And, apart from any subsidence, the natural 
effect of the deepening of a river-valley by the action of rain 
and rivers is to bring about, sooner or later, a state of things 
in which the river can no longer deepen its bed, though it can 
shift it laterally. Then, as the current impinges on and 
erodes the bank on one side, deposition of the sand and gravel 
brought down in the channel of the stream takes place on the 
opposite shore, where the motion of the water is slight. And 
in times of flood the surface of the sand and gravel thus 
