210 J. C. HAWKSHAW ON THE CONSOLIDATED 
same level as the high water of ordinary spring-tides opposite the 
Arsenal quay, falling from 2 to 3 feet below that level towards the 
lighthouse. Along the outer or sea face le large blocks of the sand- 
stone which have broken off as the reef has been undermined by the 
surf. The river flows along the inner or land face with a velocity 
at the ebb during spring-tides of from five to six knots an hour, and 
the bottom deepens rapidly from this face, there being a depth of 
28 feet at low-water spring-tide at a distance of 60 feet from the 
rock. ‘The rise of the tide at ordinary springs is 7 feet. Shells still 
preserving their colour and rounded quartz pebbles are scattered 
sparingly throughout the rock. The surface is in great part covered 
by a growth of Serpule, Balan, &e., and a calcareous coating de- 
scribed at length by Mr. Darwin. The rock is very hard, yet the 
pebbles which project in places unprotected by the organic covering 
show that it has been worn away by the action of the sea-water 
charged with sand. The organic covering, although softer than the 
rock, may be better able to resist this abrading action. A piece of 
lace when laid on the surface of a piece of glass will effectually pro- 
tect the glass from the disintegrating action of the sand-blast machine, 
the result being that the pattern of the lace will be faithfully repro- 
duced on the glass. 
The surface of the rock is marked by grooves, which have been 
described by Mr. Darwin* as follows:—‘‘There are also many 
sinuous cavities 2 or 3 inches in width and depth, and from 6 inches 
to 2 feet in length. The upper edges of these furrows sometimes 
slightly overhang their sides; they end abruptly, but in a rounded 
form. One of the furrows occasionally branches into two arms, but 
generally they are nearly parallel to each other, and placed in lines 
transverse to the beach sandstone ridge. I know not how to account 
for their origin, without they are formed by the surf, as it daily 
breaks over the bar, washing to and fro pebbles in depressions 
originally only slight. Opposed to this notion is the fact that some 
of them are lined with numerous small living Actinee.” I think the 
explanation suggested above as to the origin of the grooves is the 
true one, but that they were formed under different conditions of 
the reef, that is, when the surface was at a lower level than at 
present, so that the surface of the rock was in one plane with the 
surface of the foreshore. Under the present condition the pebbles, 
which are not abundant on the rock, would not remain long on the 
surface, and the action of the surf broken by the rough edge and 
fallen blocks on the seaward face would be very irregular. On the 
foreshore at Dover I have noticed grooves of a similar description in 
the surface of the chalk which has a regular slope from the base of 
the cliff to low-water mark. ‘These may be seen below the Castle 
Cliff to the eastward of the last groyne in that direction. Beyond 
this groyne, which is a very high one, forming a jetty raised above 
high-water mark for a great part of its length, the foreshore pre- 
* Phil. Mag. vol. xix. 1841, p. 258, and reprinted in Appendix to Darwin’s 
‘ Coral Reefs,’ 2nd edit. 
