Geology of Coalbrook Dale, 449 



as they afford a clue to the strata, the detritus of which has served to form the 

 deposit. In a quarry about half a mile W.N.W. of Shiffnall, I found rolled 

 frag-ments of the following rocks imbedded in a red, finely brecciated mass of 

 the same materials. 



White and red quartz ; 



Igneous rocks, similar to those constituting the Wrekin and Lilleshall hills; 



Carboniferous limestone, resembling that of the Lilleshall Red limestone ; 

 and 



Sandstone and ironstone, apparently from the coal-measures. 



To these conglomerates succeeds a thick series of marls and sandstones, 

 which belong to the uppermost divisions, but which do not come within the 

 range of the district described in the paper. 



The lines of joints cannot be easily recognized in this rock. A good case, 

 however, occurs at Houghton, near Shiffnall, where several beds of the coarse 

 conglomerate, above mentioned, are traversed by well-defined joints, ranging^ 

 E.N.E. ; and it presents great additional interest, from the marked illustration 

 which it affords of the powerful action of the agent which produced the joints. 

 The majority of the imbedded pebbles, some of which are six to ten inches in 

 diameter, belong- to rocks of great hardness, sometimes laminated, sometimes 

 not, the matrix being the most friable portion of the stratum ; the joints, never- 

 theless, traverse the beds perpendicularly, dividing them as with a knife, and 

 pass through all the imbedded pebbles within their range. In any other 

 fracture the strata present a studded surface.* 



ROCKS OF IGNEOUS ORIGIN. 

 (Plates XXXV. and XXXVI., figs. 1, 2, 5, 6, 7, 9, 13 and 15.) 

 In so disturbed a district as Coalbrook Dale, igneous rocks may be expected 

 to abound. It has been stated that the coal measures and older deposits flank a 

 great igneous axis, the effects of which are described among the '' lines of dis- 

 turbance." The elevated ridge of the Wrekin and Ercal is a portion of this axis, 

 ranging S.W. and N.E. The nucleus of the Wrekin consists of a very hard, 

 splintery, reddish gray hornstone,and that of the Ercal of a felspar porphyry or 

 eurite, composed of light flesh-coloured compact felspar, inclosing a few small 

 grains of colourless quartz and crystals of red common felspar. These rocks do 

 not exhibit any indications of columnar structure. They are surrounded by a 



* At Admaston, a saline spring rises in the new red sandstone, near a mass of trap ; and the 

 brine spring at Kinley-wich is similarly situated. Dr. Townson, describing the latter spring, says it 

 yields 4000 to 5000 gallons in the twenty-four hours. It is an impure brine, but was formerly 

 used. It flows out of reddish sandstone, resting upon a reddish chert like that of the Wrekin. 

 See Townson's Tracts, p. 179. 



