PASSAGES FROM VOLCANIC GRIT TO AMORPHOUS TRAP EXPLAINED. 277 



Although the volcanic grits or bedded trap rocks occupy linear ridges in this tract, and amor- 

 phous masses throw off the former in various directions, it is not always practicable to separate 

 with precision these two classes ; in fact the one is occasionally seen to graduate into the other. 

 Thus on the west and south-western face of the summit of the Corndon, the greenstone graduates 

 upwards into a thinly laminated rock, dipping to the west at an angle of about 35°, which has been 

 long worked for flagstones, the quarries having been opened to a depth of thirty to forty feet. The 

 lowest strata have a decided trappean aspect, fold in large conchoidal flakes over the underlying 

 greenstone, and from their composition must be considered as a sort of greenstone slate. These 

 flaggy beds, splitting to a thickness of three to four inches, are of a dull grey colour, have somewhat 

 a talcose and saponaceous feel, and a rough mechanical cross fracture. In the highest parts of 

 the quarry, or further removed from the trap, it is difficult to distinguish them from sandstone flags. 

 I could detect no organic remains in these beds, and I came to the conclusion that, though pro- 

 bably of an earlier date, they must, like others previously mentioned, have been formed during 

 submarine volcanic eruptions. Stratified trap of precisely the same composition, occurs in a similar 

 position on the south-western face of the greenstone of the Squilfa. {Dysgwylfa.) 



These examples of transition from amorphous and massive, into slaty and bedded 

 trap, seem to me to be highly valuable in illustrating one of the processes by which 

 the rocks of this tract have been formed ; for when the submarine volcanos were first 

 in activity, they would naturally give rise to accumulations of finely levigated scorise, 

 mud and sand, which would necessarily fold in layers round the edges of the bosses 

 and cones of more solid igneous matter* Beds so formed would therefore be presented 

 to us as one of the last links in the approach of volcanic grits towards true trap rocks ; 

 and such I conceive to have been the origin of the flagstones of the Corndon and Squilfa, 

 which fold round the promontories, and pass into the intrusive greenstone. 



Succeeding eruptions, of ephemeral character, but continued during long periods, 

 would -account for the numerous alternations of regularly bedded volcanic grit and 

 marine sediment we have described ; while, during a third aera of great disturbance, the 

 amorphous trap being thrown up to great heights upon the same parallel lines of fissure, 

 would break up and heave the stratified rocks into those anticlinal and synclinal lines 

 by which this district is so beautifully diversified. 



Principal Mining Ground. 



The most productive mining ground of this tract lies around the village of Shelve, 

 which, although about 800 feet above the level of the sea, is still in a depression, being 

 flanked on three sides by hills composed of the trap and altered rocks already de- 

 scribed, and on the fourth by the lofty, serrated ridge of the Stiper Stones. The 



petitions of the stratified courses of trap before described. The deposits in contact with the chief masses of 

 intrusive trap in the southern part of the district are often much altered and veined, particularly in the hollows 

 between the Roundtain and Taudley, where they contain nests and veins of sulphate of barytes, crystallized 

 carbonate of lime, quartz and traces of lead ore. 



2 M 



