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JOURNAL OF THE PLYMOUTH INSTITUTION. 
portance. By disregarding them, a very great risk is incurred of 
ultimate failure. We must not, on the other hand, forget that 
the very existence of such rules tends to point out a remarkable 
uniformity of the rocks in this district; at any rate, as to the 
parallelism of water-bearing fissures broadly. 
I alluded above to manufacturers in Plymouth, tanners and 
brewers mostly, requiring large supplies of water. This question 
has still for them a considerable pecuniary interest ; and when 
extensive supplies are known to be within certain proximate limits, 
such as the water-bearing fissures at North Hill ; the masses of 
dunstone, or other close rock, at Mannamead; or, as we shall see 
by-and-by, along and near the line of junction of shillat and lime- 
stone, a strong temptation to save a heavy rate, and gain an 
independent supply, is natural. It is needless to point out the 
immense direct value to the town of every large spring or well, 
especially when used for manufacturing purposes. 
A great prejudice against springs and wells supplying towns has 
existed now for several years ; and most valuable supplies are 
overlooked, mainly because some little investigation and special 
knowledge are needed to bring to light, and estimate rightly, these 
underground sources. In the present state of our knowledge 
generally on this point, we can still call a preference for reservoirs, 
filter-beds, &c. a logical weakness. Lately much has been said in 
favour of subsoil-water in reference to the future supply of the 
metropolis. The moral to provincial towns, as well as to London, 
is, "Do not neglect natural supplies in preference to a more im- 
posing machinery ; but utilize both. Encourage wells of the 
better class ; but only of the better class." No more fruitful 
source of disease has been done away with than small surface- 
wells in towns. Even now the mortality of many villages in 
Devonshire is higher than it ought to be, and these ill-placed 
surface-pits ealled wells are accountable for most of this extra 
mortality. 
Dunstone crops in very abruptly through the shillat on the 
north and north-west, as on Mutley Plain, just under the College, 
and in Eore Street, Stonehouse. At Mutley, the change is asso- 
ciated with a large bed of yellow clay of great age. Shillat is 
generally altered near dunstone, and much loosened. Mannamead 
is interlaced with dunstone, and in two conditions; mostly it is 
a hard, sound, homogeneous rock, seldom fractured, and perfectly 
