1916] on The Search for New Coal-Fields in England 627 



bordered on one side or the other by newer formations, and may, 

 therefore, each be associated with a concealed field. The resufts 

 which have been obtained np to the present in proving these exten- 

 sions and in locating coal-fields not associated with any visible coal- 

 field are shown by the black-and-white rnling on the grey ground. 



The proved extensions are indicated by a black ruliiag. One of 

 the largest is that which underlies parts of Yorkshire and Nottingham- 



strata newer than Coal Measures W.'^ 



Coal Measures WIM 



Do extensions ililiilli' 



Strata older than Coal Measures 



Do extensions LJJJ 



Fig. 1. 



shire ; but in the Cheshire Basin, also, a concealed coal-field of greafc 

 size could be shown in association with the visible coal-fields'-of 

 Lancashire, Cheshire, Shropshire, and Denbighshire, were it not for 

 the natural limit to the practicable depths of mines. On this subject 

 much evidence was taken by the Royal Commission on Coal Supplies, 

 with the result that the precedent of the Coal Commission of 1871 

 was followed, and 4,000 feet was adopted as the limit of practicable 

 depth in working. Of scarcely less importance are the proved and 



2 T 2 



