THE HYDROGEOLOGY OF THE PLYMOUTH DISTRICT. 107 
so great has been the denudation that it is now a very difficult problem 
to identify recurvences due to folding back, if any. As pointed 
out by Mr. Worth,* several northerly dips occur beyond Mannamead, 
but in no wise to balance the southerly inclination. There are, 
however, a number of secondary contortions, and it is these which 
render cuttings in shillat, as it is locally called, very uncertain. 
The extreme form of these contortions, is a mass of rock more or 
less spherical, with laminations twisted most irregularly. These 
are met with from four or five to forty or fifty feet in diameter ; 
and as the adjacent strata are frequently much displaced and broken, 
subsoil water lodges in and around them. This accounts for their 
tendency to fall, especially in wet weather. 
Shillat is called "blue," "red," or " white," although the white 
is in reality of a greenish tint. This colouring, near Plymouth, 
coincides with the plane of cleavage generally, and the bands vary 
from inches to many feet in breadth. A very broad blue band may 
be seen in the railway cutting at Harwell Street. Shillat is 
closely packed, with very few transverse fissures ; those bearing 
water are found between and in the plane of the coloured bands, 
and when open generally filled with a dull white spar. This spar, 
by no means confined to the planes of cleavage or to the larger 
fissures, darts through the rocks in every direction, and is fre- 
quently met with in large masses, with fissures converging from 
all points to the central mass. 
Lipson Hill is known to be plentifully supplied with under- 
ground water from top to bottom ; yet a well sunk by Mr. Pyper 
near the top had to be abandoned, not a drop of water being found. 
The well was 28 feet deep, and within less than 20 yards a most 
abundant supply exists. It so happened that this well came on a 
deep bed of red shillat, close, sound, and perfectly dry ; no change 
of ground was come to, and none was anticipated. 
Well- sinkers here have it, that to come on red shillat is a bad 
sign, and that there is less chance of water in red than in either of 
the other colours. This suspicion is only a comment on the general 
soundness and individuality of red shillat; which in this neigh- 
bourhood can be traced for miles on the surface, the quality also 
being kept up. It must therefore exercise no small influence on 
the underground water supply of the district. The fact of no 
water existing in it is the converse of the fact that there is a 
* "Geology of Plymouth," Trans. Plym. Inst., vol. v. p. 455. 
