76 



CLASSIFICATION OF THE PUBLIC LANDS. 



or that lies in the middle of a group of thinner beds. It is clear 

 that no two shafts that penetrate groups of coal beds will present 



Thickness of coal bed in feet 



I 2 3 ,4 5 



Figure 3.— Diagram showing deptli limits of coals of different B. t. u. and of different 



Ihlclcnesses under 6 feet. 



similar conditions. The coals of such groups will vary in number 

 from two to twenty or more and may be included in an interval 



