ERUPTIVE AND METAMORPHIC ROCKS. 5/5 



at Downhead on the Mendip Hills (felstone containing much magnetite and a 

 little hornblende), and at Stoke Lane (devitrified porphyrite with a devitrified 

 base). Volcanic breccia occurs at Wrington Warren.' At Adscombe, near 

 Nether Stowey, there is a compact fragmentary rock, possibly volcanic ash 

 (as suggested by Mr. F. Rutley), which has been worked for building purposes. 

 Syenitic rock ("Pottle stone") occurs at Hestercombe, near Taunton- (see 

 p. 141) ; and intrusive felstone is met with near Bittadon, in North Devon.^ 



At Washfield, north-west of Tiverton, is a peculiar volcanic ash considered by 

 De la Beche to have been formed during the accumulation of the Triassic series by 

 volcanic ejections falling into the water in which the sands and other detritus were 

 being accumulated. Referring also to certain large masses of quartz-porphyry that 

 occur in the New Red breccias of South Devon, and which do not distinctly 

 resemble any rocks found in sitil in the district, he thought they also might have 

 been ejected from volcanic vents and mingled with the sediments of the New Red 

 series.* Basaltic rocks, sometimes amygdaloidal, occur more particularly at or 

 near the base of the Red rocks at Posbury, south-west of Crediton, Thorverton 

 (Thorverton stone) near Silverton, at Dunchideock and Pocombe (Pocombe stone), 

 south-west of Exeter. 



Near Newton Abbot and Totnes there are many exposures of igneous rock 

 amidst the Devonian slates : some appear to be intrusive and others are contempo- 

 raneous, and they are frequently marked by hilly features, the result of their 

 resisting denudation more than the Devonian slates. The Ashprington Series of 

 Mr. Champernowne (see p. 135) includes many volcanic rocks. ^ 



In Babbacombe Bay there is intrusive dolerite ;'^ and similar rock occurs at 

 Black Head, near Torquay. Yarner Beacon, near Darlington, is formed of 

 gabbro. 



Brent Tor, near Tavistock, is perhaps one of the best preserved of our volcanic 

 vents, inasmuch as the basal portion of the original cone remains. (See Figs. 

 94, 95.) De la Beche originally described the main features of this district, 

 suggesting that there was the site of an old volcano : the rocks have since been 

 described in detail by Mr. Rutley.'' The eruptions probably took place in early 

 Carboniferous times, during the deposition of the Culm-measures, for the lavas 

 are interbedded with strata that are most likely of this age, if they are not 

 Devonian. 



Granite is largely developed in Cornwall and Devon : it is usually of a grey 

 colour, and often contains schorl. The Cheesewring granite is quarried under the 

 hill on which the Cheesewring is situated, near Liskeard, and was used in the con- 

 struction of the London Docks, Westminster and Waterloo Bridges, etc. Granite 

 also is quarried at Lamorna Cove, west of Penzance, Penryn, Falmouth, Carn 

 Brea, St. Austell, Par, Hensborough,^ Bodmin, Gready near Lostwithiel, Calstock, 

 Padstow, etc. It is extensively used for building-material. 



The Roche-rocks between Bodmin and Truro are on the borders of the granitic 

 area. St. Michael's Mount is formed of granite, while the tidal isthmus is formed 

 of slates. Porphyritic granite occurs at Lanlivery and Luxulyan (Luxullian or 

 Luxulion), south-west of Lostwithiel. The rock, which has been named 

 Luxulyanite (Luxullianite), is a porphyritic rock, consisting of large crystals 

 of pink orthoclase-felspar, and small crystals of quartz, in a ground-mass of schorl, 

 or black tourmaline. Prof. Bonney regards it as a peculiar metamorphic form of 



1 F. Rutley, in Geol. E. Somerset (Geol. Surv.), pp. 208-212 ; J. Slade, 

 P. Geol. Assoc, vii. 112 ; Moore, Q. J. xxiii. 451 ; H. H. Winwood, Proc. Bath 

 Nat. Hist. Soc. iii. 90. 



2 L. Horner, T. G. S. iii. 348. 



3 Bonney, G. Mag. 1878, p. 207. 



* Proc. G. S. ii. 198; Report Geol. Cornwall, pp. 199, etc. 

 ^ S. Allport, Q. J. xxxii. 423 ; De la Beche, Proc. G. S. i. 33. 

 ^ See W. A. E. Ussher and A. Ciiampernowne, P. Geol. Assoc, viii. 442, 458. 

 ' F. Rutley, Q. J. xxxvi. 293 ; and The Eruptive Rocks of Brent Tor, 1878 ; 

 De la Beche, Report Geol. Cornwall, etc., pp. 51, 70. 



^ See J. H. Collins, The Hensbarrow Granite District, 1S78. 



