228 SECRETS OF EABTH AND SEA 



enterprise. Anothejr ^eflectj.Qn of some ini,pprtai:ce is that 

 these world-wide sources of rock^iojl or petroleuti) are 

 likely to be exhsmsted by exploitation much sooner than 

 are the coal-fields of the world. W« cajnnot rely on their 

 long duratioxi. 



