116 JOURNAL OF THE PLYMOUTH INSTITUTION. 
stone, somewhat rich in coarse veins.' ' The most casual observation 
on the ground will at once settle this process of cementation from 
the appearance of exposed fissures. 
In Pomphlett quarries the rocks are very close ; no springs are 
met with, and little water circulates in the rocks. At low-water 
strong streams issue from the toe of the wharf wall opposite the 
quarries. These springs are salt when the tide leaves them, 
but become fresher as they run ; proving the existence of a small 
circulation from the surface. Semi-salt springs, such as the above, 
are very common on the limestone coast in this district. At West 
Hoe quarries a large semi -salt spring bubbled up from a fissure in 
the limestone, quite six inches in diameter, and, like the Oreston 
and Pomphlett springs, varied with freshes and tides in its com- 
position. Similarly, when laying the Stonehouse sewer at the 
Millbay Docks, between the life-boat house and the shipyard, about 
nine feet below the present beach, and four feet below low-water 
level, a spring perfectly identical with that in the West Hoe quarry 
was struck. This stream penetrates a dyke of fine yellow clay 
about 18 feet thick, running in a southerly direction, and is of 
considerable importance, for it undoubtedly has something to do 
with the main natural drainage of the Stonehouse district. 
The Stonehouse cavern is the central feature in the natural 
drainage of the Stonehouse district. Dr. Geach gives a very 
detailed description of this Cavern in an interesting letter to Lord 
Mount Edgcumbe, dated Plymouth Dock, March 1st, 1776, and 
reprinted in vol. vii of the "South Devon Monthly Museum," p. 153. 
Three distinct wells are here enumerated, and Dr. Geach gives an 
explanation of the discharge of the subterranean waters into 
Millbay which is correct, at least is borne out by other and inde- 
pendent evidence, such as the spring on the foreshore cited above. 
Again, Old Barrack Street well (Hare's brewery), now in the Marine 
Barracks, seems to afford us an intermediate link in tracing this 
outlet from the cavern to Millbay. This well is altogether 200 
feet deep, 50 feet shaft, and 150 feet boring. Water was tapped at 
185 feet from the old surface, and rose suddenly to within 30 feet 
of the ground, and niaintained this level. At this point (185 feet 
below surface) the rods suddenly dropped several feet, and the 
boring ended in a red standstone similar to that at Mount 
Edgcumbe. The boring was in a stratum of slate at a high 
angle, and enclosed on both sides with limestone. 
