42 TUNNELING 



The above diagram gives in compact form the classifica- 

 tion of tunnels according to materials penetrated and methods 

 of rxravation adopted, which have been described more fully 

 in the preceding paragraphs. It may be noted here again that 

 this is a purely arbitrary classification, and serves mostly as a 

 convenience in discussing the different classes of tunnels with- 

 out confusion. 



