81 
land is of trap, which rests on a red sandstone, similar to that of 
the slab. The sandstone covers some acres northward of the head- 
land at low water, running out in a succession of ridges in a north- 
east direction, and dipping to the south-east. The slab is of a tri- 
angular form; its length is 17 inches, greatest breadth 9, and the 
thickness varies from 1^ to 2 inches. It rested on the sand, with 
the honeycombed face undermost, without any other fragment's near 
it, and 20 yards, or more, from the nearest sandstone in situ. On 
one side it is dimpled with twenty-three round cavities, all cup-shaped. 
A few are so faint as to present merely slight markings ; the others 
are from 1 inch to 1 \ inch in diameter, and from i to \ an inch in 
depth, with rounded edges. The cavities shew a tendency to a linear 
arrangement; those in one line having their sides in contact, or nearly 
so ; and the whole are in two groups, the cavities in each crowded 
together, and no one cavity standing entirely detached. The oppo- 
site side of the slab has no cavities, and is thinly covered with 
some species of vegetation, apparently a lichen ; but there are four 
or five small vestiges of cavities, and two large ones on the edge of 
the slab. In explanation of the cavities, it was stated that there are 
many marine animals which bore holes in stones to make lodgments 
for themselves, such as those described in Forbes and Hanley’s 
“ British Mollusca,” and in Doctor Johnston’s “ Conchology,” 
under the titles of Pholas, Saxicava, Petricola ; and in the English 
Cyclopaedia, under that of “ Lithophagidae,” including also the Pa- 
tella and Echinus. These animals burrow in wood, chalk, limestones, 
shale, and some of them in red sandstone. Reference was also 
made to Dr Johnston’s work (Letter 10th) to show that the 
property of burrowing in stone had been attributed to some terres- 
trial mollusks (the “ Helix nemoralis,” and “ Helix aspersa”), by 
Dr Buckland, and others, though not on very certain grounds. Al- 
lusion was also made to the mode in which the Helixes named 
“ cluster together” on the under surface of stones or ledges of rock, 
as presenting an analogy with the clustered or grouped disposition 
of the cavities, on what appears to be the “ lower surface” of 
the slab, as indicated by the absence there of the vegetation seen on 
the opposite surface. The slab had evidently been moved from its 
original site, probably by the action of the waves in a storm. He 
inclined to the opinion, that the cavities were made by animals like 
the Patella, which bore holes to a moderate depth, and being perhaps 
VOL. IV. 
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