186 



TRAP-DYKE OF BARTESTREE, NEAR HEREFORD. 



contain much felspar and a little quartz, the exterior being coated with a substance 

 having a greasy feel, probably serpentine. The rocks through which the dyke passes, 

 consist of red sandstone and marl, belonging to the cornstone or central division of 

 the Old Red System, and the beds at this spot are very nearly horizontal. The 

 hard trap rock having been cut away, these horizontal strata are left on either side, 

 their edges forming vertical walls, which were in contact with the dyke. We are 

 here presented with an excellent illustration of the effects produced by intrusive trap, 

 in altering or indurating the contiguous sandstone and marl through which it has 

 passed ; for a purple amygdaloid, with kernels and nests of yellowish calcareous spar, 

 forming the outer coat of the greenstone, is demonstrably nothing more than the spotted 

 marls so altered by the action of heat, that they resemble trappean amygdaloids. The 

 distinct separation of the calcareous from the argillaceous matter, and the diversity of 

 the colours give to this rock a very peculiar aspect. The effect of these alterations is 

 seen to penetrate beyond the amygdaloid several feet into the adjoining beds. In the 

 sandstone, the grains of sand, the associated calcareous and argillaceous matter pro- 

 bably serving as a flux, have been fused into white quartz, much lime being diffused 

 through the adjacent masses. Some of the carbonate of lime occurring in the nodules 

 and veins, may doubtless in this, as in other cases, have been formed by infiltration, 

 subsequent to the intrusion of the trap ; and I only mention the prevalent diffusion of 

 calcareous matter through the mass, to indicate how essentially it may have contributed 

 as a flux in the conversion of the rock. A few concretions in the altered marl are 

 coated with crystals of pearl spar ("chaux carbonatee magnesifere primitive") arranged 

 in globular forms. Other crystals have the aspect of dolomite, and thin films of anthra- 

 cite appear here and there in the interstices, the volatile parts having been driven off 

 by heat. At a few yards from the wall of the dyke, the strata of Old Red Sandstone 

 resume their ordinary characters, and are nearly horizontal. The direction of this 

 dyke of greenstone, from west-south-west to east-north-east, is remarkable in being 

 at right angles to the axis of elevation of the adjoining Woolhope valley. It seems, 

 however, to lie in the prolongation of the elevated mass of Ludlow Rocks called Shuck - 

 nell Hill, distant three miles from this spot (see Map) ; and we may therefore infer 

 that the dislocation of the strata of that hill was connected with a line of disturbance 

 proceeding from the chain of the Malvern and Abberley Hills to Bartestree (see De- 

 scription of Valley of Woolhope) . 



Trap-Dyke, Brockhill, Worcestershire (PI. 36. fig. 2., and vignette-head of this 



chapter). 



This trap-dyke, so remarkable in mineral structure, has been exposed by quarries 

 opened in a low hill close to the left bank of the river Teme, about eleven miles north- 

 east of Worcester, and distant only one mile and a quarter from the western flank of 



