102 
THE PERMIAN VOLCANOES 
BOOK VII 
rest immediately on the Old Eed Sandstone. Two sheets of columnar 
olivine-dolerite, possibly originally connected, lie as cakes on the summit 
and eastern slope of the ridge, and cover in all a space of about a square 
mile and a half. The larger sheet, which varies from 60 to 180 feet in 
thickness, overlies the Coal-measures, and the coals of the Cornbrook coal- 
field have been worked underneath it. The smaller mass, which may 
be 300 feet in thickness, forms the summit of the ridge. On its eastern 
side it reposes on Coal-measures, which are there much disturbed ; but on 
the west side, where it forms a bold capping to the escarpment, it is under- 
lain at once by the Old Eed Sandstone. There cannot be any doubt that 
Fig. 232. — Diagrammatic section across Titterstone Glee Hill. 
1. Old Red Sandstone ; 2 . Carboniferous Limestone ; 3 . Millstone Grit ; 4. Coal-measures ; 5 5. Columnar olivine- 
dolerite. 
these masses of eruptive material are sills, which have been injected into the 
Carboniferous strata, and partly between these strata and the Old Eed 
Sandstone. One or more dykes of eruptive rock have been met with in 
mining, and the coal on approaching them undergoes alteration. 1 
2. Broum Clee Rill consists of two outliers of Coal-measures, each about 
a mile long, placed on the summit of a broad ridge of Old Eed Sandstone, 
and rising to a height of 1800 feet above the sea. Both of the outliers is 
capped with a cake of dolerite, and a third smaller patch of the same 
material lies on the southern outlier between the cappings. Neither at this 
locality nor around Titterstone Clee have any eruptive rocks been observed 
rising through the older strata. It is evident that in both cases the orifices 
or fissures up which the molten material rose lie concealed under the sur- 
viving cakes of dolerite. 2 
3. Forest of Wyre Coal-fielcl . — 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. 3 
1 See .T. R. Wright, Trans. Geol. Soc. (2nd ser. ) iii. (1832), p. 487. Titterstone Clee Hill is 
shown on Sheet 55 N.E. ancl N. W. of the Geological Survey, and in Horizontal Sections, Sheets 
33 and 36, from which Fig. 232 is reduced. The microscopic structure of the dolerite has been de- 
scribed by Mr. Allport, Geol. Mar t. 1870, p. 159; Quart. Journ. Geol. Soc. xsx. (1874), p. 550. 
1 Brown Clee Hill is mapped in Sheet 61 S.W. of the Geological Survey, and its structure is 
shown in Sheet 36 of the Horizontal Sections. 
3 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 
