G!)4 
imbedded in a lower part of the carboniferous series than 
the Rowley Range, is upon an average about two feet thick, 
and consists of a white felspathic igneous rock — of an 
earthy structure — hard when first extracted, but crumbles 
to pieces upon exposure to the atmosphere. For a mile 
or two from Wolverhampton it continues about the same 
thickness, and lies underneath a coal bed known in the 
district as the " Bottom Coal." It then breaks through 
this bed of coal and continues for a short distance lying 
immediately above it. It then swells out into a large knob 
or boss, throwing off one or two small veins into the superior 
strata, and causing two small faults therein. From thence 
in a westerly direction, with its usual thickness, until it 
reaches Powk Hill, where it has swelled out to a great 
thickness, and in cooling assumed the peculiar columnar 
forms we find there. In the course of their investigations, 
these gentlemen discovered that these beds of injected igneous 
rock were generally thicker at their edges than in the rest 
of their course. I think this may be accounted for in the 
following manner : — That portion of the igneous matter 
which was at the edge would have travelled farthest from 
the vent or fissure through which the whole mass was 
injected, and would, consequently, from having come in 
contact with a greater extent of strata of a lower tempera- 
ture than itself, have cooled down below the rest of the 
mass. 
Now, when the power which impelled the molten mass 
onwards ceased to act, a reaction would ensue, and, from 
the enormous pressure of the superincumbent strata, a 
portion of it would be forced back again ; but the edges 
being cooled, and, therefore, partially consolidated, would 
retain their bulk undiminished. 
Powk Hill is a portion of one of these edges, which, in 
cooling, has not only assumed the columnar form, but has 
