(sometimes called Hob and Jack), seven to ten feet thick, making its appearance between the "Foot coal" and the "Slips coal." These facts prepare us for the statement, of the truth of which good proof can now be given, that the change in the grouping of the beds was originally much more complete, even within the limits of the coal-field, and that the beds of coal which, coming together in the southern part of the field, form the Thick coal, are the very same beds, or some of them, which are worked now at Essington and Wyrley, and are there all more or less separated from each other by interposed shales and sandstones, so as to make a total thickness of over 300 feet. This change, from a thickness of thirty feet of coal only, to one in which thirty feet of coal are distributed among 300 feet of shales and sandstones, can be shown to have taken place within a horizontal distance not exceeding five miles. It is a kind of change quite familiar to those who are accustomed to trace any set of beds continuously along their strike, though it is not often that it can be so clearly proved and followed out in detail as in the present instance. Two of the coals below the Thick coal also undergo a similar separation towards the north.
Neither is this change in the grouping of the coals the only important change which occurs in the constitution of the field, for some of the lower beds of coal and ironstone which, in the part of the district between Wolverhampton and Walsall, are the richest and most important measures, dwindle away southwards towards Dudley, and farther south than that town have never been found at all in any workable form, though the measures of shale and sandstone in which they ought to lie have been pierced in search of them at several places.
These great changes in the constitution of the Coal-measures make it impossible to give any general section which shall be applicable in all its parts to any one locality. In the former edition of this Memoir this difficulty was evaded by giving, first, a list of the beds above the Thick coal, taken chiefly from the southern part of the field, and then a list of those below it, taken chiefly from the central and northern part. This plan, however, obscured the general view of the whole. It will, therefore, be better to give, first, a general section, compiled from the information to be obtained in the whole of the southern part of the coalfield from Bentley to Halesowen (Map 62. S.W.), and then another general and comparative section, applicable to the northern part of the district, from Bentley as far as Wyrley on the one side, and the Brown Hills on the other (Map 62. N.W.)
The first general section will include the mention of every workable bed of coal and ironstone in its proper place in the series, without regard to the locality in which it occurs. The second, while it agrees pretty nearly with the first in the lower measures (since each includes the Bentley district), will exhibit in the upper part the change before alluded to as occurring in the grouping of the coals forming the Thick coal, as well as some other changes in the lower beds.
