BORING LIFE 155 



to soft rocks, and calcareous rocks are always in danger 

 from the chemical borers, like the Date-mussel, but if 

 these types of rock are avoided there is no danger. More- 

 over they work much slower and penetrate far less deeply 

 than do the wood borers Cement and concrete are seldom 

 attacked by them, though there are a few recorded cases 

 of these substances being bored into by Piddocks or their 

 relatives. 



