190 
THE GEOLOGIST. 
It is clear, then, that the natural physical features of a 
district must be observed, if we wish to pursue our work of 
drainage effectually. 
Having thus, by means of the natural irregularities of sur- 
face, been shown the means of collecting the surface waters 
of a country, and profiting by the kindly suggestions of 
nature in guiding us in our subsequent modifications, it re- 
mains for us yet briefly to examine the processes by which 
the distribution is effected. So long as the valleys (by means 
of which the surface waters are carried off) are of such a na- 
ture that they resist the action of water, so long is the water- 
course complete ; but as this is not always the case, and their 
course runs over porous soils, an absorption of the water 
takes place, or in other words, we have a natural distribution. 
Supposing, for instance, that the course was in a clay stratum, 
non-absorbent in its properties, we should find the natural 
drainage complete, so long as the waters were carried in the 
bed of this formation ; but immediately that a change took 
place in this respect, and a porous stratum occupied the posi- 
tion where a non-porous rock had heretofore been, the waters 
thus collected would fall from the surface of the non-porous 
stratum to the subterranean water level in the porous rock, at 
whatever depth that might be. Exam.ples of this we find 
are frequent, and their detrimental effects may be observed 
in very remote, as well as proximate localities to the point 
of absorption ; of these we shall elsewhere give proofs. 
This preliminary inquiry introduces two other points, which 
although seemingly self-apparent, we think it better not to 
pass over without remark ; — that is to say, the nature of the 
porous and non-porous rocks : — for upon this, in conjunction 
with our previous statements, we shall found the greater por- 
tion of our suppositions ; and they may be said to suggest 
the feasibility of a general system of drainage, whereby the 
