Upper 

 Gallery". 



Wall-car e 



6 and 7. 



236 



FELSFATIIIC AND 



Forms part of a line of granitic rock extending from 

 Trcburget, near St. Teath, nearly to Padstow Harbour. 

 Map 30. It cuts through a series of variegated slates. 



Employed for building and for roads. Large quarries 



are opened on the St. Kew Hills. — St. Kew Hills, St. Kew, 



Cornwall. 



97. — Outer portion of a granitic dyke, 



showi 



ing a 



porphyritic arrangement 



of the same substances, and 

 the igneous origin of the granitic 



beautifully illustrating 

 matter of the dyke ; that portion which necessarily took the 

 longest time to cool, being most crystalline, while the ex- 

 ternal portions, cooling more rapidly, were less crystalline, 

 and partook more of the porphyritic structure. 



The exterior portions of this dyke, being not so readily 

 worked with the tool, are not so much used for common 

 architectural purposes as its interior portions ; though, 

 where expense is no great object, the ornamental parts 

 of a building would be far more durable if made of its 



•Near St. Coluinb Minor, Corn- 



outer portions. Map 30. 



wall. 



98. — Porphyritic rock, greyish-white quartzo-felspathic 



base, containing imperfect crystals 



felsp 



plates of brown mica. Map 30. — Helstone, near Camel- 

 ford, Cornwall. 



90. — Felspar porphyry, a fine grained compound of 

 felspar and quartz with crystals of mica. Forms part of an 

 elvan dyke, known as the Pentuan elvan or Pentuan stone. 



182 and 183.) Map 31. 



(S 



on 



•Pentuan, St. Austell, Cornwall. 

 100. 



■f/< 



decomposed, with a small quantity of quartz, and a little 

 silvery mica in a greenish-coloured felspathic base. Form 

 part of an elvan dyke. Map 31. (See "Report on Corn- 

 wall," p. 177.) — Crecgbroaz Quarries, near Chacewater, 

 Cornwall. 



101. — Felspar porphyry. — Part of a granitic vein, pro- 

 ceeding from the main body of granite to the eastward of 



sei 



ff 

 no 



103- 



<tr i 



101.- 



frtt, t 

 ' . Fi 



tie adj a- 



tank 



In the 



a 1 . 



• 



wegnlti 



( lay con 





be 

 d be 



10L 



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