THE GEOLOGY OF PLYMOUTH. 
459 
can at once account for the alternations of slate with calcareous 
shale observable on each side of the limestone. Whichever form 
of detritus predominated for the time, governed throughout its 
sway the character of the current deposit. At intervals silt and 
sand were cast upon the reef itself. Thus we account for the slates 
and sandstones (in part) which occur in beds or patches between 
the layers of limestone. Some of the sand-beds may be of later 
date, and attributable, as at Torquay, to Triassic sandstone filling 
up the joints produced by natural forces in a later age. 
The depth to which our strata have been proved exceeds 300 
feet. This was in the boring for the Victoria Spa, in Bath Street. 
The section was — 
Feet. 
Earthy clay slate . ... 20 
Limestone 
Blue slate 
Ked sandstone 
Limestone 
Sandstone 
Ked and blue slate 
Dunstone 
Earthy clay slate 
Red sandstone 
150 
20 
3 
50 
4 
30 
8 
20 
12 
The slate appeared to come in in wedges. Allowing for dip, 
the actual thickness of the beds traversed would certainly not 
exceed two-thirds of these amounts. 
There is sandy as well as slaty limestone. Treating a reddish- 
coloured arenaceous limestone from the Hoe with dilute hydro- 
chloric acid, I found that 25 per cent, of its bulk consisted of a 
bright-red siliceous sand, exceedingly fine. The stone was of the 
kind known to the quarry men as " hard head," which occurs 
more or less intermixed with the limestone proper throughout its 
range. 
The limestone forms a band half a mile in width, and nearly 
6^ miles in length. Its main western extremity is in Devonport 
Dockyard (there are detached beds on the other side of the water 
further west than Empacombe). It breaks off on the east about 
Sherford. Its general elevation is much lower than that of the 
slate hills, averaging about a hundred feet ; and whilst its summit 
line has been truly described as being level as a wall, its continuity 
