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XXXV. — Notice of the Discovery of a mass of Trap Rock in the Mountain 

 Limestone of Bleadon Hill, in the County of Somerset. 



By the Rev. D. WILLIAMS, F.G.S. 



[Read June 10th, 1840.] 



X FEEL much pleasure in announcing to the Society, that a cutting for the Bristol 

 and Exeter railway through the extreme western termination of Bleadon Hill has 

 within these few days disclosed a mass of fine porphyritic trap, the first instance 

 hitherto observed of the occurrence of an igneous rock in the Mendip Hills, or 

 within the area of the Bristol coal-field. The trap varies from a granular green- 

 stone with hornblende to a compact porphyritic rock, inclosing amygdaloidal ker- 

 nels of calcareous spar and brown hematite ; other specimens exhibit crystals of 

 glassy felspar, and the rock is sometimes laminated and friable. The bed of 

 mountain limestone which rests immediately on the trap differs in colour, and is 

 much more brittle than the other strata. 



Rare as such igneous products are in this region, they are scarcely less so 

 throughout the whole of Exmoor and the Quantocks ; for, except the syenitic 

 granite and trappean porphyry of Hestercomb in the Quantocks, first noticed by 

 Mr. Horner*, a slaty porphyry I met with a little north of Simmons-birth in Ex- 

 moor, and the elvan blocks near Morte Point, mentioned by Mr. De la Bechef, all 

 associated with the same slate series (No. 5. of my subdivision), which rests on 

 the granite of Lundy Island, I know of none. The next nearest igneous rocks 

 are those about Tortworth in Gloucestershire, in the Caradoc sandstone division of 

 the Silurian system of Mr. Murchison. 



The cutting for the railway intersects the broadest part of the ridge of lias and 

 new red sandstone, which I formerly pointed out J as filling up a great interval or 

 original gorge between the limestone of Bleadon and Uphill. 



* Geol. Trans., First Series, vol. iii. p. 348, 1816. 



t Report, Geology of Cornwall and Devon, pp. 49, 186, 1839. 



X Geol. Proceedings, vol. ii. p. 79, 1833. 



