158 
Forest, and unless the deeper seams of coal are won before this 
occurs, it is highly probable that the falling off in the supply 
of coal will commence in about 30 years. Much, however, 
depends, as previously noticed, upon the average rate at which 
the coal field is worked. 
When mineral elements are held in solution and suspen- 
sion, as in the water of a lake or sea, the particles having the 
greatest specific gravity generally settle in the deepest part, the 
lighter ones floating longer, and finally settling at higher 
levels, or in places of repose in the ratio of the difference of 
their specific gravity ; consequently it must depend upon the 
proof of this, or some other more rational theory of the deposi- 
tion of metallic elements, whether a general mass of iron-ore 
will be found to occupy a large area in the centre of the Forest 
of Dean Mineral Basin or not. At present the workings in the 
Carboniferous Limestone have not been carried to a sufficient 
depth to definitely prove this question one way or another. 
Formerly an idea was entertained by individuals residing 
in some parts of the Forest of Dean that the water from the 
River Wye percolated into portions of the iron-ore and coal 
field towards the North; but, considering the effect of the 
various beds of indurated shale and clay which are known to 
surround the entire mineral basin, it must be clear that such 
notions emanated from persons who had not studied or com- 
prehended the Geological features of the district, and conse- 
quently such notions could have no foundation in fact. 
All the water percolating into the belts of iron-ore and 
Carboniferous Limestone measures, as well as those belonging 
to the sandstone and to the coal measures, is derived from 
rainfall, and the quantity to be encountered in any given area 
underground can never be more than a certain proportion of so 
many rainfall inches annually falling upon a given percolating 
area at the surface, after making such deductions as may be 
necessary for areas of inclined ground, which, like so many 
inclined planes of different angles, tend to throw a considerable 
quantity of the water into the low level valleys and to the sea 
before it has had time to percolate. 
