57- ERUPTIVE AND METAMORnilC ROCKS. 



crystals of pinkish-red orthoclase. From its beautiful appearance it is extensively 

 wrought for ornamental purposes. It is of special interest from the number and 

 wide distribution of the erratic blocks whicli have been derived from it.^ (See 

 p. 487.) At Ennerdale (Pillar rock, etc.) and Buttermere there is a syenitic granite, 

 and at Carrock Fell there is feistone-porphyry. The former rock was considered 

 by Mr. Ward to be metamorphic, and likewise the quartz-felsite of St. John's Vale, 

 a rock quarried for paving, etc., at Threlkeld.- 



In the Kendal and Sedbergh districts mica-traps have been noticed, including 

 minette, kersantite, micaceous diorite, porphyrite, etc. ; these do not penetrate 

 rocks of later age than Silurian. ^ 



Mr. Ward has observed that on the coast between Castletown and Poolvash, in 

 the Isle of Man, there may be seen "a small, ancient volcano, dissected and laid 

 bare." The Volcanic rocks extend from Scarlet Point to Poolvash, a distance of 

 about a mile and a quarter, and consist almost wholly of ash and breccia, with 

 dykes of basalt. The Stack of Scarlet shows the finest development of basalt, 

 forming a dyke, which in places is about fifty feet wide ; and this intrusive rock, in 

 Mr. Ward's opinion, represents an original line of eruption.* 



Igneous rocks are largely developed in North Wales. According to Sir A. C. 

 Ramsay, west, south, and east of Dolgelly, masses of ' greenstone ' have been pro- 

 truded amid the strata, the irregular forms of the igneous rocks being in some 

 degree due to the contortions of the Lingula Flags, among which they lie. The 

 Tremadoc Slates are overlaid by great accumulations of volcanic ashes and lava 

 flows, extending in a crescent form round the Lower Cambrian, Lingula, and 

 Tremadoc beds, and forming the heights of Cader Idris, the Arans, the Arenigs, 

 etc. ; while Sir A. C. Ramsay observes that most of the rocks between Bettws-y- 

 Coed, Moel Siabod, Llyn-Ogwen, and Conway, are the actual equivalents of the 

 slender band of Bala limestone, near Bala, and of the slates and insignificant ashy 

 bands that underlie it. (See pp. 72, 73). Moel Siabod and Carnedd Llewelyn 

 show at their summits great masses of 'greenstone ' intruded amidst the fossiliferous 

 grits and slates of the Bala formation, although they sometimes coincide with the 

 lines of bedding Siiowdon, Moel Hebog, and V-(jlyder-fawr are formed largely 

 of beds of felspathic porphyry associated with fossiliferous grits and volcanic ash- 

 beds, with occasional intruded masses of ' greenstone.' (See Fig. 10, p. 68). 



Lava-flows of Bala age occur in the pass of Llanberis, etc. The Gimblet Rock 

 at Pwllheli is a Diabase.* Felstones have been noted at Aran Mowddwy, Glaslyn, 

 and Capel Curig ; but many of the so-called felstones of North Wales were, in 

 Mr. Rutley's opinion, originally lavas of a highly glassy type. Perlitic and spheru- 

 litic structures are met with in the lavas of the Glyder-fawr, North Wales, ^ and 

 spherulitic rhyolites occur at Digoed, near Penmachno.'' The spherules (litho- 

 physen) which occur in these rocks often attain considerable dimensions and have 

 been carefully investigated and described by Mr. Cole. 



Numerous bosses of 'greenstone' occur in the Lleyn promontory in North 

 Wales. The greenstone dykes which occur in the slate quarries of Penrhyn and 

 Llanberis are considered by Sir A. C. Ramsay to be of later date than the 

 contortions of the strata that produced the cleavage in them, and they are probably 

 Post-Carboniferous.* 



1 J. A. Phillips, Q. J. xxxvi. 9 ; H. A. Nicholson, Geol. Cumberland and 

 Westmorland, 1868, p. 41. 



^ Q. J. xxxii. 25 ; see also Nicholson, Q. J. xxv. 435. 



3 Aveiine and Hughes, Explan. Sheet 98 N.E. (Geol. Surv.), p. 16, 98 S.E., 

 p. 41 ; T. G. Bonney and F. T. S. Houghton, Q. J. xxxv. 179. 



* G. Mag. 1S80, p. 5. 



* This and other North Wales Rocks were investigated by E. B. Tawney, 

 G. Mag. iSSo, pp. 207, 452, 1882, p. 548, 18S3, pp. 17, 65 ; see also Ramsay, 

 Geol. North Wales, ed. 2, 1881 ; Q. J. ix. 170; Sedgwick, Q. J. iii. 135; 

 Bonney, Q. J. xxxviii. 2S9. 



* F. Rutley, Q. J. xxxv. 50S ; Felsitic Lavas of England and Wales, p. 15. 

 7 G. A. J. Cole, Q. J. xU. 162, xlii. 183. 



^ See also G. Maw, G. Mag. 1868, p. 125. 



