186 LIFE OF BEAN BUCK LAND. [ch. vii. 
His opinion on this subject was recognised as so authori¬ 
tative that his advice was often asked by practical men in 
important undertakings. The old breakwater at Weymouth 
having been much injured by the pholas boring into the 
limestone, the engineer consulted Dr. Buckland upon the 
best stone to be used in making the new one. After many 
exhaustive inquiries, he recommended that Portland stone 
(of which St. Paul’s is built) should be used, as the 
pholas will not bore into it on account of the quantity of 
silica or flinty matter it contains. In 1841 he published 
a paper in the Proceedings of the Geological Society upon 
the agency of land-snails in corroding and making deep 
excavations in compact limestone. He examined the 
peculiar hollows on the under surface of a ledge of carboni¬ 
ferous limestone rock, and, as he found in them a large 
number of the shells of Helix aspersa , he concluded that 
the cavities had been formed by snails, and that probably 
many generations had contributed to produce them. He 
intended to ascertain whether the cavities were hollowed 
out by these snails by means of an acid secreted by them, 
or by means of their rasp-like tongues. In a speech 
delivered before the Geological Section of the British 
Association at Cambridge in 1845, he discussed the ques¬ 
tion at some length. The following extract from the 
Times gives a summary of what he said :— 
“ Dr. Buckland described the agency of land-snails in 
forming holes and trackways in compact limestone. His 
attention had first been called to the subject by a discussion 
on the perforations sixty feet high at Tenby Castle, which 
were by some taken to be evidence of a raised beach, but 
