194 
THE GEOLOGIST. 
tions^ but of which ample evidence is afforded in the increased 
altitudes of the projections of Cornwall and Wales. 
Thus you have a progressive increase of altitude as you pro- 
ceed from east to west. 
This at once establishes the principle that drainage could 
easily take place from the more lofty parts of England towards 
the marine level ; and this geological definition of the com- 
parative altitudes^ and the rule for drainage thus afforded^ is 
proved by the course of the rivers being precisely analogous* 
They rise in the regions which — geologically speaking and 
a priori in reality — are the highest and fall mainly to the 
eastward, whilst a few find outlets in those picturesque water- 
courses for which Wales, Cornwall, and the north-western 
counties are distinguished. 
Let us now look at the courses of these rivers. The greater 
number among them — and these include the principal rivers 
in this country — have their courses over secondary rocks, 
generally speaking of a non-porous character : this appHes in 
a more especial degree to the navigable rivers, which are navi- 
gable only so far as their course extends over non-porous 
ground. Thus the Thames, the Ouse, the Humber, &c., run 
over ground more or less of a clayey or non-absorbent quality, 
and the point at which the oolitic or greensand strata is reached, 
is simultaneously the point at which each is liable to desic- 
cation in the warmer months, and hence become unnavigable. 
This affords another illustration — viz. — that nature provides 
for drainage and irrigation in some measure without the as- 
sistance of art. But, whereas in the earlier days of civilisa- 
tion, the inhabitants of a country locate themselves for this 
very purpose on the banks of streams, so as population in- 
creases, do they gradually find their way inland ; and hence 
the necessity of providing, artificially, for what nature had 
originally designed no support. 
But we have established the rule — that nature has laws of 
