266 



MINERAL PITCH AND VOLCANIC ACTION. 



The mass of the slaty rock near the trap o*) is very brittle and fractures into small 

 rhombs, externally dull blue, but of a greenish grey colour within. A few small flakes 

 of anthracite appear here and there with veins and nests of white quartz. At one point 

 above the nucleus of trap the quarrymen exposed and left standing (when I was last 

 there) a conical mass of slaty schist about twenty feet high, a great portion of the 

 surfaces of which was covered by bitumen, which had exuded from the interstices and 

 oozed over the inclined edges of the rock. On examining this mass more attentively, 

 I observed that the bitumen was supplied from a vertical vein or fissure, the greatest 

 width of which was about two and a half feet, and which proceeding upwards from 

 the trap terminated in a narrow channel, whence the viscid matter flowed over upon 

 the sides of the slaty rock, sometimes covering it in globular forms, sometimes in 

 black lines and films. The vein from which the bitumen exudes, consists of fragments 

 of the sandy slate itself, which are frequently cemented by the bitumen. The imme- 

 diate contiguity of the trap to the slaty rock is a most interesting discovery, and leads 

 very naturally to the belief that the same volcanic action which produced the trap may 

 have produced the bitumen, whilst it also explains the origin of similar phenomena in 

 Lyth Hill, the Longmynd, and other places in this district. The bituminous matter 

 thus occurring so frequently at points where trap rocks are intruded amid the strata, 

 might at first sight appear to support the theory of Breislack, who observing the abun- 

 dance of petroleum in the neighbourhood of volcanos, and the quantity given out during 

 their eruption, conceived that all volcanic operations might be due to the ignition of that 

 combustible in subterranean caverns, set on fire by the action of some third substance, 

 such as decomposing sulphuret of iron 1 . This hypothesis has not, however, been favour- 

 ably received by geologists, who consider such a cause to be quite incommensurate with 

 the great effects produced by igneous action. Without, however, entering further upon 

 the question of the source of volcanos, the frequent occurrence of mineral pitch in these 

 ancient rocks is undeniably a valuable link, in establishing the parallel between their 

 composition and those of modern volcanos, disposing us still more to adhere to the 

 belief of their common origin. 



As Lilleshall Hill is the extreme prolongation of the trappean line on the eastern 

 boundary, so is this of Haughmond Hill the last evidence on the left bank of the Severn 

 of the continuance of this western parallel. Both these hills of trap rock are surrounded 

 by the New Red Sandstone, the strata of which rest in positions more or less horizontal 

 upon the vertical or highly inclined edges of the older slaty rocks, and have evidently 

 been deposited subsequent to these trappean eruptions. In a subsequent chapter, how- 

 ever, it will be shown that other trap rocks (on the axis of the Breidden Hills) have 

 cut through the New Red Sandstone. 



In respect to the course of these ridges of trap rock in Shropshire, we have to remark 



' See Daubeny on Volcanos, p. 357. 



