SYENITIC TRAP-DYKE OF BROCKHILL, WORCESTERSHIRE. 



187 



the Abberley Hills. It cuts through the Old Red Sandstone in a direction from five 

 degrees north of west, to five degrees south of east ; and being followed on its course 

 towards the Abberley Hills, has been laid open to a depth of about forty feet. The trap 

 is in one part about eight paces wide, and near its walls puts on a prismatic form, the 

 edges of the joints of the columns decomposing precisely like those of the basalt of 

 the Giant's Causeway, and some very regular columns have been extracted. The com- 

 position of the rock is unlike any I had previously seen in basaltiform dykes. It is in 

 fact partly a dark green columnar syenite, made up of hornblende, flesh-coloured felspar 

 and quartz, undistinguishable from varieties of the Malvern syenite ; and partly an 

 amygdaloid, containing kernels of carbonate of lime, which on exposure weather out 

 and leave cavities on the sides of the prisms. A "sahlbande," a few inches thick, of a 

 dark green colour, having a greasy feel, and being partially amygdaloidal, is interposed 

 between the trap and the altered rocks. The sections on each side of the dyke exhibit 

 horizontal beds, consisting of regular alternations of flaglike, micaceous sandstone and 

 shale, with several courses, from one to four feet thick, of concretionary, spotted, 

 impure limestone or cornstone. In contact with the trap, and for twenty feet and more 

 from its walls, the sandstone is much hardened, mica is wanting, and the colour of the 

 rock is changed to a dark purple. The variegated marls and cornstone are converted 

 into an indurated mass like that of Bartestree, resembling many trappean amygdaloids: 

 the lime is disseminated in veins and coatings of white crystallized carbonate of lime, 

 with a few crystals of iron pyrites. Other varieties of these altered marls have a splintery 

 conchoidal fracture, are partially prismatized, and have lost all traces of lamination. 

 As this dyke is analogous in composition to some of the rocky prolongations of the 

 Malvern ridge which extend to near Abberley 1 , and as its course points directly to 

 Woodbury, the most prominent of those elevations, we obtain one of the proofs that 

 volcanic forces have been in activity along that great fissure of eruption subsequent 

 to the consolidation of the Old Red Sandstone. Again, from the highly dislocated con- 

 dition of the patches of coal, which adhere to the flanks of these hills, no doubt can 

 remain, that volcanic action was also continued upon this line, after the deposit of the 

 carboniferous system. 



With the evidence we shall hereafter adduce of the frequency of trappean eruptions 

 during the formation of the Silurian system, and with the proofs we have already given 

 of the outburst of such rocks subsequent to the consolidation of the coal measures, 

 Chapters 5 to 1 1 , it is surprising, that during the accumulation of the widely expanded 

 series of Old Red Sandstone, there should apparently have been a total cessation of the 

 evolution of igneous matter ; for the dykes we have just described must have been in- 

 truded after the old red strata were deposited. It is further well to remark in this place, 

 that the prevalent horizontality of the great masses of Old Red Sandstone, in the counties 



1 See Map and subsequent Chapter on the Malvern and Abberley Hills. 



