155 
The Farewell Rock or Millstone Grit, of marine origin, 
occurs at the base of the lower coal measures in the South 
Wales coal basin. In fact, the lowest seams of coal in the 
White Ash series rests immediately upon it; then succeeds in 
depth the Millstone Grit series. The beds of coal correspond- 
ing most in position in the Forest of Dean appear to be the 
Trenchard seams, which are situated not far above the Mill- 
stone Grit series, but whether there is any analogy between 
the elements contained in the series referred to in the two 
districts, has not, as far as I know, been attempted to be proved. 
Doubtless it would be exceedingly difficult to form a 
proper basis for correlation, but if it is ever attempted in a 
serious manner a larger number of accurate sections must be 
formed in each mining district, the petrographical character of 
all the beds of rock, shale, and clay determined, as well as an 
exhaustive chemical analysis of all the coal seams in each 
district. Such a work would be of great interest and value, 
supposing no definite correlation could be made. Probably it 
would be most convenient to take a well-defined point in the 
Mountain Limestone, or other rocks above corresponding in 
the various districts from which vertical measurements for a 
series of sections should be taken for correlative purposes. 
It is probable that the middle and Southern portions of 
the Welsh coal field were much depressed, whilst that portion 
of it to the North-East and the Dean Forest may have been 
much more elevated, and consequently exposed to the severest 
effects produced by denuding and other natural influences long 
before a general equilibrium, and the present line of surface 
resulted. 
The upper part of the coal field of Dean Forest must con- 
sequently have been swept away during the epoch when the 
anti-clinal or higher ridges of the Carboniferous Limestone 
appeared, and were broken up, which also had been planed 
down and remained in its present condition. 
It is natural to infer that there were a series of anti-clinals 
and syn-clinals, producing a series of basin-like forms, the 
lowest part of which would consequently have been more or 
E2 
