THE HYDROGEOLOGY OF THE PLYMOUTH DISTRICT. 113 
line, the shillat becomes suddenly soft, and merges into a yellow 
clay in which thin layers of limestone appear, all within 30 feet. 
At other points we find alternate beds of shillat and limestone, as 
near Clarendon Place ; also a bastard rock, as at Devil's Point and 
Millbay. "Water is found plentifully all along the line of division, 
and in the limestone where bastard rock or slate occurs. 
The Corporation Cesspit between Queen Anne's Battery and the 
Bopewalk is 20 feet deep in bastard rock, here associated with 
tapering beds of yellow clay. Abundance of water was found when 
excavating. This pit is near the division line. Polio wing this 
line westwards, the well in Mr. Scott's brewery is the next of 
importance near it ; here strata of slate alternating with clay are 
cut to a depth of 48 feet. Little or no limestone was met with so 
far as I can discover, but it is just outside the division line between 
the slate and limestone. Water is found in great abundance, of 
most excellent quality, and evidently in communication with an 
extended subsoil system, for during severe droughts it keeps up 
well. Several years ago this well was cleaned, when strong 
springs were seen bubbling up on the floor. The supply from below 
the level of the bottom of the well seemed to be greater than from 
the sides. The rate of rise was quite 2' 6" per hour at the lower 
part of the well ; and as the shaft is 6' 6" diam., this makes the 
hourly discharge 500 gallons. 
At the Messrs. Butchers' brewery on the left of Stonehouse 
Bridge, there is a well singularly like this one in Hoegate Street. 
It too is situated a few yards north of the division line, is 86 feet 
deep, and is driven through blue slate and spar, mostly white. I 
am inclined to think this white spar is quite distinct from the 
darker coloured spars of the pure slate rock, and owes its presence 
to aqueous deposition and not to igneous intrusion, as the spar in 
Mutley tunnel cutting does. Indeed the composition of the water 
almost proves this, for it contains salts procurable only from some 
such soluble rock. Dr. Letheby found here, Sulphate of Lime, 
Carbonate of Lime, Carbonate of Magnesia, Sulphate of Soda, 
Sulphate of Potassium. A uniform temperature of 54° Fah. is 
maintained winter and summer, and a never-failing supply, which is 
drawn on day and night, for the pumps here never stop. The five 
salts named above are enough to show a general similarity in com- 
position to the waters of the Victoria Spa of the Royal Union Baths, 
formerly situated at the corner of Union Street and Bath Street. 
VOL. VI. H 
