walls of which vary from eight to
thirteen feet in thickness, whatever his progenitor may have done in
1077.

The Keep--the last resort of the garrison when all the outworks were
taken--is considered so beautiful that it is selected, under the article
"Castle" in the last edition of the _Encyclopaedia Britannica_, as an
illustration of Norman architecture, showing "an embattled parapet often
admitting of chambers and staircases being constructed," and showing
also "embattled turrets carried one story higher than the parapet."
There is also a fine woodcut of the Castle at p. 198 of vol. v. of that
work.

The Keep is seventy feet square and a hundred feet high, built of the
native Kentish ragstone and Caen stone; and the adamantine mortar or
cement used in its construction was made with sand, evidently procured
at the seaside some distance from Rochester, for it contains remains of
cardium, pecten, solen, and other marine shells, which would not be
found in river sand. Mr. Roach Smith suggested that probably the sand
may have been procured from "Cockle-shell Hard," near Sheerness. He
called our attention to the fact that in Norman mortar sand is
predominant, and in Roman mortar lime or chalk.

[Illustration: Rochester Castle]

The roof and the chambers are gone,--the Keep remains as a mere
shell,--and where bishops, kings, and barons came and went, flocks of
the common domestic pigeon, in countless numbers, fly about and make
their home and multiply. One almost regrets the freedom which these
graceful birds possess, although to grudge freedom to a pigeon is like
grudging sunshine to a flower. But though the damage to the walls is
really trifling, as they will stand for centuries to come, still the
litter and mess which the birds naturally make is considerable and
unsightly, and decidedly out of keeping in such a magnificent ruin. The
pigeons exhibit what takes place when a species becomes dominant to the
exclusion of other species, as witness the pest of the rabbits in New
Zealand. With profound respect to his Worship the Mayor and the
Corporation of Rochester, to whom the Castle and grounds now belong, the
writer of these lines, as a naturalist, ventures to suggest that the
Castle should be left to the jackdaws, its natural and doubtless its
original tenants, which, although of higher organization, have been
driven out by superior numbers in the "struggle for existence," and for
whom it is a much more appropriate ha