486 



We are also indebted to Mr. Murcliison for some interesting 

 remarks on the dislocations of the strata in the neighbourhood of 

 Dudley, and particularly for a description of some dome-shaped 

 masses, from the centre of which the beds have a quaquaversal dip. 

 He speculates on the probable dependence of these phsenomena 

 upon the protrusion of volcanic matter from below, at points where 

 it has been unable to find issue. It would, I think, have been more 

 satisfactory, if, in confirmation of his theory, some natural section of 

 one of these dome-shaped masses could be pointed out, where not 

 only a nucleus of trap was apparent, but could be shown to have 

 taken up its actual position in a soft or fluid state. Even if we 

 should find in some instances a subjacent central mass of trap, por- 

 phyry or granite, not sending out veins or altering the strata, the 

 folding of the beds round such a protuberance might admit of an 

 explanation like that suggested by Dr. Fitton. He has supposed 

 a set of yielding horizontal strata to be pressed upon by a sub- 

 jacent hill or boss of hard rock, in which case the effect of upward 

 pressure might resemble that seen, on a small scale, in the paper of 

 a bound book, where a minute knob in one leaf has imparted its 

 shape to a great number of other leaves without piercing through 

 them*. Whatever hypothesis we favour, 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 everywhei'e outwards from a common centre; but, in the vol- 

 canic mountain, the beds having an outward dip, thin off as they 

 approach the base or circumference of the cone, which is not the 

 case in inclined beds composing the hills alluded to in the neigh- 

 bourhood 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 instance of the volcanic 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 sur- 

 rounding low country. 



* Dr. Fitton, Gcol. Trans. 2ud Series, vol. iv. p. 244. 



