Vol. 60.] IGNEOUS ROCKS OF THE BRISTOL DISTRICT. 1^7 



12. The Igneous Rocks associated v:lth the Carboniferous Limestone 

 of the Bristol District. By Prof. Conwy Lloyd Morgan, 

 LL.D., F.R.S., F.G.S., and Prof. Sidney Hugh Reynolds, M.A., 

 F.G.S. (Rend December 16th, 1903.) 



[Plates XVI & XVIL] 



Content.-. 



• Page 



I . Introduction 137 



II. The Evidence for the Contemporaneous Origin of the Igneou> 



Kocks 139 



III. The Approximate Horizon of the Igneous Bocks 147 



IV. The Petrology of the Igneous Bocks 151 



(A) The Lavas. 



(B) The Tuffs. 



V. Conclusions 155 



I. Introduction. 



In the ' Summary of Progress* of the Geological Survey for l v '.*^ 

 (pp. 1<>4-11) Sir Archibald Geikie & Mr. Aubrey Strahan contri- 

 buted an admirable section on the Volcanic Group associated with 

 the Carboniferous Limestone of Xorthern Somerset. Although in 

 this summary the evidence for the contemporaneous character of the 

 igneous rocks is clearly set forth, the subject is of sufficient interest 

 and importance to justify some further record. 



The earliest-published reference to these volcanic rocks, with 

 which we are acquainted, occurs in a note contributed by the 

 Rev. D. Williams to the Geological Society on June 10th, 1S40. 1 

 The note refers to the occurrence of ' fine porphyritic trap ' in 

 the Uphill Cutting, near Weston-super-Mare, of the Bristol &' Exeter 

 (now Great Western) Railway-line. In the figure which accom- 

 panies Williams's short paper the igneous rock is described as • trap, 

 apparently substituted for the originally continuous limestone, by 

 slow fusion and conversion.' Dean Buckland appears, however, to 

 have observed the occurrence of igneous rocks at another locality in 

 the district as early as 1817, though the first-published record ot 

 the fact occurs in his Presidential Address to the Somersetshire 

 Archaeological & Natural History Society in 1849. 2 Although no 

 detailed evidence is given, his brief statement suggests that he had 

 recognized the volcanic nature of the beds. ' The vents." he says, 

 'that have discharged igneous rocks in the hills of Somerset are 

 few," and adds that one of these occurs 'on the N.W. shoulder of 



1 Trans. Geol. Soc. ser. 2. vol. vi. pt. ii (1842 p. 561. 



2 Proc. Somerset. Arch. & Xat. Hist. .Soc. vol. i 1851 p. 18. 



