THE HYDROGEOLOGY OF THE PLYMOUTH DISTRICT. Ill 
impervious to water ; occasionally it is in a spongy state, and 
bearing plentiful traces of iron. 
The shaft of a well at Lamorna Yilla, Mannamead, through 
close and sound dunstone, was 15 feet deep, and perfectly dry. 
Below this, rods were used, and when boring at a depth of about 
30 feet from the surface, the tool suddenly dropped a small 
distance, and quite a rush of water followed ; indeed several of 
the tools in this well could not be removed, simply on account of 
the sudden rising of the water. Several instances of unsuccessful 
wells in the close dunstone can be cited, all showing that un- 
fractured dunstone is a perfectly impassable barrier to subsoil-water, 
and that only where it is fractured, or in a spongy condition, can 
an outlet be effected. At Castle Compton House, Mannamead, in a 
well 21 feet deep, with a heading at this level 12 feet long in a 
north-east direction, spongy dunstone was exposed, and penetrated 
at the end of the heading. Water came freely immediately this 
spongy rock was struck, and in a short time filled the well to 
within four feet of the surface, where it remained. It was sub- 
sequently determined to cut further into this open rock, hoping 
thereby to secure a still greater supply ; but two men, with two 
eighteen gallon casks mounted on a windlass, could not keep the 
water under. Both these wells are but slightly affected by weather, 
and show that the dunstone by disturbing the shillat, perfectly 
intercepting the subsoil-water — or, as with the spongy, providing 
a clear channel for the passage of the water — very much extends 
the natural collecting limit of the undisturbed shillat. 
In a fissure where the frictional resistance to the water is con- 
stant, or, in other words, where the filtering materials in the 
fissure are uniform in size, the water will have a definite and 
uniform gradient. "Where they are not uniform, there will be 
a series of gradients, precisely as in the well-known experiment 
in hydraulics.* 
This explains how water may be had long after rain, and far 
above the outlet in open soils. "We have a small example of this 
in Lipson Hill, where water is found at the very summit, and in 
tolerable abundance. 
The discharge of subsoil-water is fixed by the direction of 
porous planes or other openings. As we have seen, these are very 
* See " Theory of Stream Lines." By Wm. Froude, f.r.s., at Brit. Asso., 
1875 ; " Nature," December 2nd, 1875., fig. xx. p. 91. 
