104 
THE PERMIAN VOLCANOES 
BOOK VII 
the conditions under which they were erupted, and in particular to their 
comparative thickness, and the influence of adjacent coals and carbonaceous 
shales upon them. 
The igneous rocks seen at the surface in this district form a series 
of well-marked eminences. Of these the largest extends as a ridge from 
I )udley to beyond Eowley Eegis, a distance of more than two miles. To the 
west of this tract, a number of small patches of the same material crop out 
at the surface, the most important forming Barrow Hill. Six miles farther 
north another group of similar patches may be seen. Of these the largest 
occurs at Wednesfleld, but the most noted forms the Pouk Hill, which lias 
long been noted for the beauty of its columnar structure. 
The sheets of “ greenstone ” met with in the coal-field are more numerous 
and extensive than the detached areas of more compact rock visible above 
ground, a single sheet being sometimes traceable in the coal-workings for 
two miles in one direction. 
The eruptive rocks of this district, when examined in their freshest form, 
consist of well-preserved olivine-dolerite. An examination of the “ green- 
stone ” and the “ white rock-trap,” which runs in fingers and threads through 
the coal, shows that these are really the same dolerite which has undergone 
alteration, the ferruginous silicates having especially been decomposed. 1 
The sills of greenish decomposed material that have been injected amongst 
and alter the coals, vary from 15 feet to 80 or 90 feet in thickness. The 
largest of the dolerite cakes on the surface, that of the Eowley Hills, is 
somewhat irregular in its thickness, but may reach as much as 100 feet. 
That nearly the whole of the igneous material is intrusive is admitted 
by all observers who have studied the ground. The manner in which the 
“ basalts ” and “ greenstones ” send out veins into the Coal-measures shows 
conclusively that they have been injected into the strata. The only rock 
about which some doubt has been expressed is that of the Eowley Hills, 
which Jukes was disposed, though not without some hesitation, to consider 
as part of an actual lava-stream. He based this inference chiefly on the 
occurrence, immediately under the dolerite, of what he looked upon as a 
“ trappean breccia or breceiated ash, containing rounded and angular frag- 
ments of igneous rock lying in a brown rather ferruginous paste, that looks 
like the debris of a basaltic rock.” 2 This breccia he regarded as belonging 
to and passing into the Coal-measures, and he was thus inclined to regard 
the dolerite as a lava of Coal-measure age. 
It is possible, however, that the “ trappean breccia ” may be of the same 
nature as the “ uncompressed balls of basalt bedded in a mass of decom- 
posed basalt or basaltic ! waeke ’ or clay” 3 — that is, a decayed contact 
layer of the eruptive rock. But if it be regarded as the fragmental accom- 
paniment of a lava-stream, it can hardly belong to the Coal-measures. If 
1 Allport, Quart. Journ. Gcal. Soc. xxx. (1874), p. 547. Chemical analysis also shows the identity 
of the rocks and the nature of the alteration of the “white rock.” See Jukes, “South Stafford- 
shire Coal-field,” pp. 117, 118. 
2 Op. cit. p. 119. 3 
Op. cit. p. 126. 
