60 
rOPULAR SCIENCE REVIEW. 
manufacturing populations are to be found in the first district ; 
the working people of the latter are chiefly agriculturists.”* 
The lower beds of the lias and the rhsetic beds form a gentle 
escarpment above the triassic series, bordered generally by the 
escarpment of the oolites, f which stretches from Dorsetshire, 
by the Cotteswold Hills, to the Yorkshire coast. 
The next principal feature is the escarpment of the chalk 
forming the North and South Downs, and Salisbury Plain ; 
stretching from the Chiltern Hills into Norfolk, and again ap- 
pearing in the wolds of Lincolnshire. 
The tertiary clays and sands are all of a more or less yielding 
nature. They form tracts of low lying and gently undulating 
country, in Hampshire and in the Eastern counties. 
The most recent deposits of alluvium (sediment deposited by 
existing rivers and estuaries) form tracts of almost level land, 
such as that which borders the Thames, conspicuously between 
London and Tilbury, the Fenland, and the Somersetshire 
levels. 
It will be easily understood why the alluvium should form 
such scenery as it does — belts of low-lying meadow-land border- 
ing the rivers and streams — but why the rivers should have 
formed so much sediment along their courses is not at once easy 
to understand. It must be remembered, however, that in all 
times, from the most remote to the present day, changes of level 
continually take place, and such changes greatly affect denuda- 
tion and deposition. A slight submergence would cause a river 
to flood its banks and deposit sediment, whereas elevation would 
tend to make it deepen its channel, and so remove more mate- 
rial. A river, as is well known, is incessantly changing its 
course, in however small a degree, and when we see broad flats 
of alluvium bordering a small stream, it does not follow that 
the stream once entirely filled that broad channel, but that in 
the course of time it has occupied different positions in that 
valley. 
Alluvium, which is often restricted to the river mud, may 
very properly include the beds of gravel deposited by the river. 
These beds of gravel, which are often very extensive, are seldom 
entirely formed by the river itself ; they are often made up to a 
large extent of beds of gravel previously formed. Thus, much 
of the Thames valley gravel is only derived from pre-existing 
glacial gravels, which are themselves made up in great part of 
old shingles belonging to the tertiary and triassic periods. 
* See “ Manual of Geology,” by Jukes and Geikie, p. G17. 
t This is used as a general term, to embrace what is really a series of 
minor escarpments, although, looked at in a large way, the oolites form one 
striking feature. 
