SYMMETRICAL JOINTS OF THE SILURIAN HOCKS. 



491 



beneath the present floor, justly conjecturing, that if this was really the same limestone as that at 

 Dudley, there would be a second or lower band ; and their enterprise was rewarded by discovering, at 

 a depth of fifty yards, another and thicker calcareous zone, thus completely establishing the identity 

 between the limestone on the hill sides of the Castle Hill and Wren's Nest, and this underground 

 mass, distant upwards of a mile from the nearest of these hills. 



Whether, therefore, we examine the lower edges and sides of the carboniferous strata 

 of the Dudley field as they are turned up, or penetrate through those central parts 

 where they are thickest, they are found to be incumbent upon some member of the 

 Silurian System. In certain portions of the field, the Ludlow rocks, it is true, emerge 

 from beneath them; but as we approach the Rowley Hills, the Wenlock limestone would 

 seem to be the fundamental rock. Reserving all observations upon the great lines of 

 fault and fracture of the coal-field, until the trap rocks have been described, the history 

 of these Upper Silurian rocks may be concluded by reminding the reader, that the im- 

 portant formations, of Carboniferous Limestone and Old Red Sandstone, which usually 

 accompany and support the coal-fields of Great Britain, are not merely absent, but offer 

 no traces of their ever having existed in this region ; all the phenomena inducing us to 

 believe, that the carbonaceous matter was here originally deposited upon the Ludlow 

 and Wenlock formations, or Upper Silurian rocks, 



Symmetrical Joints of the Silurian Rocks in this district. 



The Silurian rocks of this tract, whether belonging to the Ludlow or the Wenlock formations, 

 are invariably divided by symmetrical joints similar to those described in Shropshire and many other 

 localities, (p. 243 et seq.) 



Without swelling the work by details, I may select the Wren's Nest as a point to illustrate my 

 views. Let the southern corner of that ellipsoid be examined, just where the strata, receding from 

 the great dislocation near " Gabriel's Hole,'" are inclined at angles varying from 25° to 30°, the 

 direction of that inclination gradually changing from S. to S.W., without the slightest break of the 

 strata. There, in an ancient quarry resembling a vaulted crypt (see left hand of vignette, p. 485.), 

 and within a space of less than 150 paces, the observer will perceive at least three distinct directions 

 of joints diagonal to the strike, the change occurring wherever there is a perceptible variation in the 

 strike of the beds. Thus, in 



No. 1. The faces forming the salient angles trend from N.N.W. to S.S.E., and from E.N.E. to W.S.W. 



No. 2. ■ ■ . . N. to S. E. to W. 



No. 3. . N.N.E. to S.S.W., E.S.E. to N.N.E. 



Other and similar changes of direction of the faces of the joints take place all round the Wren's 

 Nest with every change in the direction of the strike. I did not examine these joints with very 

 great precision, an operation which I leave to Professor Phillips, or any good geometrician who 

 will look into the subject, having myself little doubt, that the very various directions of these joints 

 will all be found to have symmetrical relations to the strike of the strata. 



