232 



GRANITIC DYKES. 



Upper 

 Gallery. 



Wall-cases 

 6 and 7. 



This dyke traverses the fossiliferous recks of the dis- 

 trict at right angles to their line of strike (see Geological 

 Maps Nos. 30 and 31), and can readily be traced from the 

 cliffs in Watergate Bay in a southerly direction through 

 St. Columb Minor, Chapel, Benewalls, and Trerice, towards 

 St. Michael or Mitchell. 



It is opened in several places along this line for quarries 

 and the central portions, being somewhat decomposed, are 



'■freestones 



30. — Near St. Columb Minor, Cornwall. 



Map 



73. 



k Jelsp 



Fine-grained sciiorlaceous granite, composed of 



and schorl. It forms a vein travers- 

 al Cornwall," p. 172.) Map 



ing serpentine. (See "Report on Cornwall," p. 172 

 32. — Kynance Cove, Cornwall. 



74. — Granitic Porphyry, from the central part of a 



dyke, and composed of small crystals of flesh-coloured fel- 

 spar, and isolated and radiating nests of schorl in a felspathic 

 base. Much employed for economic purposes. Map 31. 



"Report on Cornwall," p. 177.) — Seveock Water, 

 between Redruth and Truro, Cornwall. 



75. — Felspar porphyry, composed of crystals of light, 

 coloured felspar, with small radiating nests of schorl and a 

 little quartz, in a felspathic base. Part of a granitic dyke. 



Map 



•Near St. Hilary, Cornwall. 



76. — Felspar porphyry, composed of a quartzo -felspathic 



ffelsp 





nd schorl. The 



specimen shews the composition of the inner parts of a dyke 

 which juts out on the coast from beneath that great mass of 

 calcareous sandhills known as Piran Sands, which extend 

 from Piran Porth on the south to Penhale on the north. 



This dyke is seen to cut through argillaceous slates on the 

 south side, on the north its junction was concealed by the 

 sands from view in 1835. ~ " 



joining slate are seen among the mass "of porphyry, and 

 have, for the most part, been somewhat altered in their 

 ■character, though they have not lost their schistose struc- 



Included fragments of the ad- 



ture. Map 



Piran Sands, Cornwall. 



I 



from 1 



OH' 

 ft- 



i 



hich 



lis 



lie h 



o 



e: 



-Tn 

 81, 



taiai r 



and 



k] 



82, 



[• 



iftie 



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