BURROWERS AND BORERS. 103 



one, and the birds were constantly flying over it, but had 

 not the wit to see that it was not thick enough to suit their 

 purpose. 



It has been already mentioned that roads paved with 

 hard granite are quickly worn away by the constant passage 

 of heavy traffic, but it would hardly be imagined that the 

 passing to and fro of birds' feet could make any impression 

 on the rocks, yet in Nightingale Island, Tristan da Cunha, 

 these are actually smoothed and polished by the continual 

 tread of hundreds 01 penguins on their way to and from the 

 sea.* No doubt the flinty skeletons of the microscopic 

 plants called diatoms, which are always found in abundance 

 in the mud about their nests, adhere to their feet and act as 

 polishing powder. 



Beneath the penguin rookeries are the holes of the 

 prions and petrels, which are bored in all directions, the 

 round being honeycombed to such an extent that it often 

 gives way when human beings venture upon it ; and much 

 the same may be said of the sandy flats about the Cape 

 of Good Hope, only there the burrowers are moles, whose 

 tunnels are so large as easily to admit the hand and 

 arm.f 



The boring mollusks, too, must not be entirely passed 

 over, as, although their shells are as thin as paper, and as 

 brittle as glass, they are able to pierce wood, limestone 

 clay, slate, and even sandstone, to the depth of several 



* The island is a mile square and is inhabited by about 400,000 penguins, 

 whose rookeries cover a quarter of its area. 



f In the Chilian Andes, the burrows of the little chinchilla are so 

 numerous as considerably to increase the difficulty of travelling. 



