TRAP ROCKS OF BARROW HILL, ETC. 



499 



up and deteriorated in quality, but its beds are penetrated by lateral wedges of greenstone, which 

 thin out as they recede from the chief centre of eruption 1 . 



The evidences respecting the source of the trap rocks in other parts of the district are, however, 

 still more convincing ; for other insulated masses of trap appear to the west and south of Dudley, 

 and all offer proofs of having been forced up through the surrounding coal measures. The smallest 

 of these masses is the Devil's Elbow, on the edge of the canal below Netherton ; a second occurs 

 near Russell's Hall; a third at Cowper's Bank; a fourth at the Fiery Holes; and the fifth, and 

 much the most important, is at Barrow Hill. At the Devil's Elbow, one variety of the rock has a 

 base of felspar, coloured green by chlorite, and traversed by streaks of calcareous spar; a second is 

 an amygdaloid, with large kernels of white calcareous spar ; whilst a third, apparently forming part 

 of a dyke, is a crystalline hornblende rock like that of the Hailstone. This outburst of trappean 

 matter serves to explain how the lower and unproductive coal strata have been thrown up to the 

 surface in a dome, on the northern and eastern slopes of which the lower coal measures have 

 been worked out 2 . 



Near the Fiery Holes, a low cliff about 30 feet high, of concretionary trap has been laid open, 

 and on one side of it is a thin vertical bed of altered rock in contact with that variety of trap which 

 the French term " basalte en boules," whilst all the adjacent coal measures are exceedingly frac- 

 tured. Here the prevailing trap is made up of granular felspar with small concretions of quartz, 

 the mass being tinged green by disseminated chlorite. 



Barrow Hill and its dependent rocks is, however, by far the largest and most instructive of these 

 trappean masses. It is composed partly of a greenstone, which on the decomposition of the horn- 

 blende weathers to a rusty yellow. Where most homogeneous, this rock has a tendency to a co- 

 lumnar structure ; but the chief portion of the hill consists of a dull rotten wacke, sometimes 

 amygdaloidal, with much green earth, and frequently bulging out into large concretions. In the 

 old quarries, where the harder rock or greenstone has been extracted 3 , the wacke and amygdaloid 

 being left standing, have weathered into very picturesque forms, resembling, in miniature, those 

 pyramidal masses which geologists and tourists who have visited the Hebrides may have remarked 

 on the eastern coasts of Skye 4 . One of these masses is about 50 feet high. They offer in their 

 structure the most convincing proofs of the posterior intrusion of the trap, for their sides frequently 

 exhibit fragments of the sandstone and shale twisted up, sometimes altered into hornstone ; and 

 small layers of the coal itself being broken and bent up in the mass of trap. Veins, occasionally 



1 I have to thank Mr. Best of Corngreaves for a knowledge of this fact. At Hyett's collier)'' pits, the shaft 

 passed through a thin sheet of basalt three feet thick which occupied the place of the heathen coal. 



2 By a letter received from Mr. W. Mathews, while this chapter is going through the press, it appears that 

 recent excavations have much more completely laid open the trap rocks near Netherton than when I examined 

 the spot. A trap dyke, bearing north-east, runs from the canal, sinking under Netherton Hill before it reaches 

 the church. The tunnel now in progress exposes this dyke for a width of about 60 yards. It is composed 

 of amygdaloidal trap and greenstone, with veins and altered rocks on either side, containing chalcedony and 

 calcareous spar, and on one flank a succession of inclined beds of coal measure, comprising indurated shale, 

 sandstone, and partings, coarse and fine conglomerate, with coal, shale, grit and coaly matter, 



3 Mr. Yates compares this greenstone with that of Salisbury Craigs near Edinburgh, which contains crystals 

 of augite, and also suggests an analogy between certain basalts of this spot (more or less vitreous) and the 

 pitchstone of Arran. He further notices a point of trap on Brierley Hill which escaped my observation. 



- 4 The Storr near Portree is the finest example. (See Macculloch's Western Islands.) 



