3. _Forest of Wyre Coal-field._--On both sides of this extensive tract of Coal-measures, the strata near the base of the series are traversed by sills or dykes of olivine-dolerite like that of the Clee Hills. The sandstones in contact with the eruptive rock have been indurated. In this district, also, the evidence shows that the sheets are intrusive, and later than the portion of the Coal-measures there visible.[115] [Footnote 115: This district is represented in Sheets 55 N.E. and 61 S.E. of the Geological Survey. The microscopic structure of the larger mass on the west side of the coal-field, and the variations in the minute structure of the intrusion which forms a long ridge on the east side, are described by Mr. Allport, _Quart. Journ. Geol. Soc._ xxx. pp. 550, 551.] 4. _Coalbrookdale Coal-field._--In this interesting district a sill of rather finely crystalline olivine-dolerite, which is estimated to be nearly 200 feet thick, is traceable from near Little Wenlock for three miles to the north, intercalated between the Carboniferous Limestone and the Silurian rocks underneath. It appears to underlie the western part of the Coal-field, for it is exposed by denudation in several valleys between Little Wenlock and Great Dawley. Owing to the thinning out of the Carboniferous Limestone in an easterly direction, the sill gradually comes to have the Millstone Grit on its upper surface, and at one point is represented on the Geological Survey map as even intruded into the Coal-measures. Here again we have an intrusive sheet of later date than at least the earlier part of the Coal-measures, and no evidence of any superficial outflow of volcanic material.[116] [Footnote 116: The Coalbrookdale coal-field has been described by Sir Joseph Prestwich, _Trans. Geol. Soc._ (2) v. p. 428; and Prof. E. Hull, _Quart. Jour. Geol. Soc._ xxxiii. (1877), p. 629. The minute structure of the sill at Little Wenlock is referred to by Mr. Allport, _op. cit._ p. 550. The ground is mapped on Sheet 61 N.E. of the Geological Survey, and its structure is shown on Sheet 54 of the Horizontal Sections.] 5. _South Staffordshire Coal-field._--This district, in respect to its igneous intercalations, has been much more fully examined and described than any of the others. It forms the subject of an exceedingly able memoir by Jukes, who carefully studied its geology and delineated it on the maps and sections of the Geological Survey. Since his time the rocks have been studied microscopically, but no material facts regarding the stratigraphy have been obtained in addition to those which he patiently collected and generalized upon.[117] [Footnote 117: Jukes, "South Staffordshire Coal-field," _Mem. Geol. Surv._ 2nd edit. (1859). The area is embraced in Sheet 62 N.W. and S.W. of the Geological Survey, and is illustrated in Sheets 23, 24 and 25 of the Horizontal Sections.]