THE HYDROGEOLOGY OF THE PLYMOUTH DISTRICT. 115 
analysis of the water by Prof. W. Herapath, sen., f.c.s., of Bristol 
Laboratory, who has appended analyses of the waters of the two 
principal Burton brewers, with which Plymouth compares most 
favourably. 
Polkinghorne Allsopp 
Bass and 
and Co. 
and Sons. 
Co. 
, 11*96 
7-248 
2-72 
1-32 
3-360 
10-56 
10-40 
7-200 
2-56 
4-16 
0-480 
1-60 
3-68 
. a trace 
0-118 
24-64 
25 080 
54-88 
Sulphates of Magnesia and Soda . 
21-600 
8-96 
Total Grains in Imperial Gal. . 
. 54-08 
65-286 
83-36 
Generally the most important wells in the whole district are in 
or near this division line, between the slate and limestone. I 
have included in the limestone area those wells which, though still 
in slate, are near the limestone, and affected by its contiguity. To 
determine the localities from which these wells are supplied is a 
work of much interest, but of very great difficulty. That their 
area of absorption is large cannot be doubted ; for, unlike the 
London basin, we hear of no diminution in the better-known wells- 
There still remain several belonging to this class well worth 
examining; but more time cannot be devoted to them in the 
present general sketch. 
In the limestone district proper, in Pomphlett quarries, Oreston 
and Deadman's Eay quarries, where the rock is pure, no water is 
met with. This is due mainly to the very definite nature of the 
fissures, and their extensive ramifications in connection with the 
tidal waters ; any water finding its way underground is conducted 
directly to the sea level. I said any water ; for in limestone there 
is a natural process of cementation at work — the deposition of 
carbonate of lime as stalactite, by which veins ^and fissures are 
sealed up, only the very pronounced openings remaining. 
Mr. Worth * thus quotes Mr. Pengelly on the Oreston caves : 
" Moreover he found that such portion of the roof of the cavern 
as remained was a mass of limestone breccia, made up of large 
angular fragments, cemented with carbonate of lime, and easily 
enough mistaken, without careful inspection, for ordinary lime- 
* " Geology of Plymouth," Trans. Plym. Inst., vol. v. p. 470. 
H 2 
