Geological Society of London. 83 
of other leaves without piercing through them.* Whatever hypoth- 
esis we favor, it is essential to observe that such hills as the Wren’s 
nest near Dudley, and others of similar ellipsoidal forms and internal 
structure, do not correspond to the type of volcanic hills, such as 
Etna, Mount Dor, or the Cantal. In both cases there may be an 
approach to a cone, and the beds may dip every where outwards 
from a common centre; but, in the volcanic mountain, the beds 
having an outward dip, thin off as they approach the base or circum- 
ference of the cone, which is not the case in inclined beds composing 
the hills alluded to in the neighborhood of Dudley; nor in the last 
mentioned instances do the lowest or subjacent rocks crop out round 
the circumference of the cone, as happens in the instances of the vol- 
canic eminences before alluded to, where the granite of the country 
round Mount Dor, the fresh-water beds and mica schist in the Cantal, 
the marine deposits around Mount Etna in Sicily,—each appear at 
the surface as soon as we have left the slope of the cone, and advance 
upon the surrounding low country. 
In attempting to explain the principal transverse faults of the 
Dudley coal-field, Mr. Murchison refers frequently to the theoretical 
principles expounded by Mr. Hopkins in his Researches in Physical 
Geology, a paper printed in the sixth volume of the Transactions of 
the Cambridge Philosophical Society. Mr. Hopkins has there en- 
deavored to develop, by reasoning founded on mechanical principles, 
and by mathematical methods, the effects of an elevatory force 
acting simultaneously at every point, beneath extensive portions of 
the crust of the earth. He’is aware that in nature such a force must 
usually act under complicated conditions, so as to produce irregular 
phenomena; but he observes that in order to have a clear concep- 
"tion of the manner in which it would operate in producing move- 
ments and dislocations, it is useful to assume certain simple condi- 
~ tions to which mathematical investigations may be applied. When 
we have deduced in this manner some results free from all uncer- 
tainty, these may serve as standard cases to which the geologist may 
refer more complex problems. Thus for example, a portion of the 
earth’s crust may be assumed to be of indefinite length, of uniform 
depth, and bounded laterally by two vertical parallel planes, beyond 
which the disturbing force does not extend. It is then supposed 
that a quantity of subterranean vapor or melted rock, existing at a 
* Dr. Fitton, Geol. Trans. 2nd Series, vol. iv. p. 244. 
