626 Mr. Aubrey Strahan [March 17, 



The term " visible coal-field " is applied to those tracts where the 

 coal measures crop out at the surface and are open to view except 

 where they are covered by superficial materials such as gravels, 

 boulder-clay, etc. These are the tracts which are shown as coal-fields 

 on ordinary geological maps. But the coal measures occupy a 

 definite position rather low down in the sequence of geological 

 formations, and may be overlain by a variety of newer groups of 

 strata, ranging from Permian upwards. A tract of coal measures 

 overlain by any newer formation is known as a " concealed coal- 

 field " : it is not indicated on ordinary geological maps, but is left to 

 the intelligence of the observer to discover. 



Obviously he must note, in the first place, what is the character 

 of the limits of a visible coal-field as shown on the map. If the coal 

 measures terminate because older strata rise from beneath them and 

 usurp the surface, the limit, as shown, definitely includes all the area 

 in which coal should be sought. If, on the other hand, the area 

 shown as coal measures adjoins an area occupied by newer formations, 

 then the mapped boundary indicates the limit of those newer strata, 

 but not necessarily the limit of the coal measures. The map forming 

 Fig. 1 has been constructed to illustrate the position of visible 

 coal-fields in relation to older and newer formations. Upon it all 

 areas occupied l)y formations older than coal measures are left white ; 

 visible coal-fields are shown in black ; and areas occupied by newer 

 formations are shown in grey. In the area left white search for coal 

 is useless, though it has not on that account been abandoned. It 

 was said years ago, and is probably still true, that the money expended 

 in boring for coal in places where no coal could exist would suffice 

 to pay the annual cost of the Geological Survey. 



From the areas coloured black nearly all the coal of past years 

 has been raised, and these are the sources the exhaustion of which is 

 justifiably regarded as being within sight. 



The areas coloured grey are those in which there is no a priori 

 reason why coal should not occur. Somewhere within these areas He 

 the concealed coal-fields. 



For example, in North "Wales a visible coal-field extends through 

 Flintshire and Denbighshire. Westwards older formations rise to 

 the surface, and the coal-field is definitely terminated ; eastwards the 

 coal measures pass beneath New Pied Sandstone, and form a concealed 

 coal-field, of which the existence is certain, but of which the extent 

 and accessibility have still to be proved. The South Wales coal- 

 field, on the other hand, is almost wholly surrounded by older 

 formations, and though some of it may lie at an inaccessible depth, 

 only an insignificant part of it is " concealed " in the sense expressed 

 above. In other ])arts of the country we find that the visii)le coal- 

 fields of Cumberland, Durham and Northumberland, Yorkshire with 

 Nottinghamshire and Derbyshire, Staffordshire, Shropshire, Warwick- 

 shire, Leicestershire, and Somerset with Gloucestershire, are all 



