218 PEOF. S. H. REYNOLDS ON A SILURIAN [May I907, 



The field-exposures were mapped by the officers of the Geological 

 Survey, Messrs. H. Bristow, W. A. E. Ussher, & H. B. Woodward ; 

 and, as mentioned in the Survey-memoir, 1 Mr. Ussher considered 

 the exposures to be parts of one continuous mass, while Messrs. 

 Bristow & Woodward considered that they occurred as bosses or 

 hummocks in the manner represented in the Survey-map. A 

 diagrammatic section by Mr. Ussher is given, 2 which shows the 

 igneous rock as a dyke breaking across the Old Eed Sandstone, 

 and spreading out near the surface of the ground. The Old Red 

 Sandstone is represented as considerably disturbed. 



Sir Archibald Geikie and Dr. A. Strahan, 3 in their account of 

 the igneous rocks associated with the Carboniferous Series of the 

 Bristol District, briefly describe the rocks in question as follows : — 



* A group of masses of igneous rock is shown on the Geological- Survey map 

 (Sheet 19) rising along the ridge of Old Eed Sandstone to the north of 

 Shepton Mallet. These masses were also visited by the Director-General and 

 Mr. Strahan, with the view of ascertaining whether they formed part of the 

 Volcanic Series which has now been described. 4 But they were found to 

 belong to a different petrographical series. Thin slices prepared from the 

 freshest of the rocks in Moon's-Hill Quarry and from Beacon Hill have been 

 examined by Mr. Teall, who finds that they are undoubted andesites. They 

 appear to rise intrusively through the (Upper) Old Bed Sandstone. They 

 have none of the characters of true lavas, nor do they seem to have any 

 accompaniment of tuff or agglomerate. Their age must remain undecided. 

 They are presumably younger than the Old Bed Sandstone, which in their 

 immediate neighbourhood passes conformably upward into the shales and 

 limestones at the base of the Carboniferous system. They may possibly belong 

 to the Carboniferous volcanic series. Their more acid character than that of 

 the lavas of Weston, Middle Hope, and Goblin Combe would not be a valid 

 objection to such a relationship, for the intrusive sheets and bosses of the 

 Carboniferous series in the South of Scotland and in Ireland are often much 

 less basic than the contemporaneous lavas. This question, however, must 

 remain until the district is surveyed in more detail.' 



Hitherto all geologists have agreed in regarding the rocks as 

 intrusive ; but, during the last few years, the opening of several 

 new quarries pointed to the desirability of remapping the exposures. 

 This was done with more interesting results than might have been 

 anticipated, as it showed that the trap-rocks were associated with 

 an extensive series of tuffs, some of which contained Silurian 

 fossils. The mutual relations of the different rocks were, however, 

 very difficult to ascertain, as much of the mapping was based on 

 material thrown out by moles and rabbits. For the further 

 elucidation of the geology of the district a Committee of the 

 British Association was appointed at the recent meeting (1906) 

 at York ; and, under the auspices of the Committee, a series of 



1 ' Geology of East Somerset & the Bristol Coalfields ' Mem. Geol. Surv. 1876, 

 p. 14. 



2 Ibid. p. 15. 



3 ' Summary of Progress of the Geological Survey for 1898 ' (1899) pp. 110-11. 



4 Namely, that associated with the Carboniferous rocks of Northern 

 Somerset. 



