IO4 



THE WORLD'S LUMBER ROOM. 



inches, and thus frequently destroy the foundations of jetties, 

 sea-walls, &c., and ultimately cause their destruction. 



Plymouth breakwater, which is constructed of hard 

 marble-like limestone, was so much injured by their ravages 



Fig. 2i. PHOLAS IN A SHELTER HOLLOWED BY IT IN A BLOCK OF 



GNEISS. 



that it was found necessary to replace the blocks between 

 high and low water-mark by granite, against which they are 

 powerless. 



Somewhat similar excavations are made by a species of 

 land-snail ; but this is believed by Dr. Buckland to work 

 with its rasp-like tongue, whereas the pholas seems to have 

 no other tool than its papery shell. (Fig. 21.) 



