106 JOURNAL OF THE PLYMOUTH INSTITUTION. 
On the smallest reflection it rrmst be evident that there is a 
geography of lakes and rivers invisible, yet real, and well-nigh 
coextensive with their more familiar contemporaries on the surface, 
containing water, as a rule, of a superior quality, and from a more 
reliable source — the absorption of large areas. Obviously this 
Hydrogeology depends on geology primarily for its features; the 
various pervious and impervious strata, faults and fissures, are its 
factors, and from these its lakes, rivers, and fountains or springs 
arise. The lower valley of the Thames is a well-known example 
of such an underground lake, or series of lakes, one above the 
other, each defined or enclosed by an impervious stratum, in this 
case London Clay. When first tapped by artesian wells, an abun- 
dant, indeed quite overpowering, supply was in most cases had. 
As well by well was sunk in the same basin, the head in the older 
wells got reduced, wells half a mile distant affecting one another. 
The permanent lowering of the London basin proved two very 
interesting points : First, that the supply stored was very much 
less in quantity than was previously supposed ; and, by the way, 
this has been also found to be true of the Upper Chalk subsoil- 
water in Kent. Secondly, it expelled the idea of similarity to 
surface lakes or rivers. The permanent surface of these lakes 
need not be a plane, certainly not horizontal. It is true, when 
undisturbed by extraordinary accidents, such as fissures, wells, or 
porous strata, the top of the subsoil-water will approximate to a 
plane, either with a gradient or level. 
One other general remark. Fissures are now admitted to be the 
main agents of transmission in the older rocks; even the Upper 
Chalk has no available supply for wells other than that due to 
fissures. When these are filled (as in the Plymouth district) with 
porous materials, we have a combination of laws due to the two 
distinct rocks, pervious and impervious. Generally, a system 
dependent on fissures ; particularly, a delivery dependent on a 
porous material. 
This district, for our purposes, is divisible into two areas, with 
little in common : I. The Shillat and Dunstone District ; II. The 
Limestone District. 
I. The Shillat and Dunstone District. 
Commonly speaking, the planes of cleavage, and also of stratifi- 
cation, near Plymouth, dip south and south-east at a high angle ; 
