ERUPTIVE AND METAMORPHIC ROCKS. 573 



The Penmaenmawr stone near Caernarvon is a quartziferous Diorite or Enstatite- 

 Diabase, which, from its toughness, forms a valuable paving material, and is 

 extensively used for buildincj.purposes and road-making.' It occurs as an intrusive 

 mass in the Upper Cambrian (Ordovician) rocks near Llanfairfechan, in Caernarvon- 

 shire. 



The occurrence of boulders of hornblende-picrite has been noted at Pen-y- 

 Carnisiog, Anglesey, in Caernarvonshire, and near St. Davids.- 



In Anglesea the green ' Mona Marble ' of Rhoscolyn, Holyhead and neighbour- 

 ing coast, is a serpentine.'* 



In the neighbourhood of Old Radnor the Woolhope Limestone is disturbed and 

 altered by eruptive rocks, and especially near the Stanner Rock, east of that 

 town.* This Rock, according to Mr. Cole, is composed of graphic granite, 

 passing into pegmatite, and in the same district he has noticed the occurrence of 

 Diabase, Quartz- Felsite, etc. Syenite occurs at Hanter Hill, south-east of Old 

 Radnor ; and the Llanelwedd stone, north of Builth, is one among several 

 exposures of eruptive rocks. 



In the Breidden Hills, north-east of Welshpool, there are many eruptive rocks ; 

 they comprise an older series of andesitic lavas, and a newer series of intrusive 

 diabases, etc.^ 



The eruptive rocks that occur in the district of St. Davids, in South Wales, 

 are among the oldest in the country. They have previously been described (see 

 pp. 37-41), hence it is only desirable to add that Dr. Hicks has brought forward 

 evidence to show that the newest of the Pebidian rocks must have been folded, 

 cleaved, and have undergone great structural changes before the Lower Cambrian 

 conglomerate was deposited. He maintains that the position of the great feUitic 

 series (Arvonian) is certainly between the Dimetian and Pebidian, but it is difficult 

 to determine whether it should be classed as of Pre-Pebidian age.^ The acceptance 

 of these views leads naturally to the conclusion that there is a more marked break 

 between the Archsean and Cambrian rocks than has been suggested previously 

 (see p. 22). 



Mr. F. Rutley has noted the occurrence of devitrified obsidians with perlitic and 

 spherulitic structures, among the eruptive rocks of Skomer Island, off the coast of 

 Pembrokeshire.'' 



Hornblendic gneiss, together with granitic and syenitic rocks, mica-schists and 

 quartzites, occur in the Malvern range, and are intersected in places by intrusive 

 dykes of pegmatite.^ (See p. 36.) 



Basaltic rocks that penetrate the Carboniferous and older rocks occur in Shrop- 

 shire and South Staffordshire. In the latter county the basalt, locally known as the 

 " Green Rock," contains a large proportion of a green chloritic mineral. It occurs 

 in sheets between the Coal-measures from the base of the Rowley Hills and Barrow 

 Hill, through the centre of the district up to Wolverhampton. Bilston, and 

 Bentley.^ The basalt or dolerite of Rowley Regis forms a hill 820 feet above 



1 J. A. Phillips, Q. J. xxxiii. 423 ; T. li. Waller, Midland Nat. viii. i ; Teall, 

 Q. J. xl. 656. 



- T. G. Bonney, Q J. xxxix. 254, xli 515 ; John Plant, G. Mag 1884, p. 48 ; 

 Teall, Brit. Petrography, Plates iv. vi. 



^ T. G. Bonney, Q. J. xxxvii. 40; Teall, Brit. Petrogr. p. 125. 



* W. S. Symonds, Records of the Rocks, 160; Murchison, Proc. G. S. ii. 85 ; 

 Cole, G. Mag. 1SS6, p. 220. 



5 W. W. Watts, Q. J. xli. 532 ; Murchison, Siluria, edit. 5, p 80. 



^ Q. J. xlii. 356 ; see also A. Geikie, Q. J. xxxix. 294, 325 ; and T. G. Bonney, 

 Q.J. xlii. 361. Prof. J. F. Blake has proposed to create a new 'system,' the 

 ' Monian,' after one place of their development (Anglesey), for the Dimetian and 

 other Pre-Cambrian rocks ; but this seems an unnecessary burden to our already 

 too copious nomenclature. Brit. Assoc. 1886. 



' Q. J. xxxvii. 409 ; see also Felsitic Lavas of England and Wales, p. 18. 



^ Rutley, Q. J. xliii. 



^ S. AUport, G. Mag. 1869, p. I15, 1870, p. 160; Bonney, Q. J. xxxii. 140; 

 Jukes, South Staffordshire Coal-field, ed. 2, pp. 120, 126; D. Forbes, (}. Mag. 

 1866, p. 25 ; J. R. Wright, T. G. S. (2), iii. 487 ; Teall, Brit. Petrogr. PI. xi. 



