Vol. 60.] IGNEOUS ROCKS OF THE BRISTOL DISTRICT. 151 



Group A and the lowest part of Group B. Here, again, therefore, 

 the horizon of the volcanic rocks may be assigned to the same 

 position as in other localities. 



(6) Westerly exposure. — The only definite piece of evidence 

 of the position of these beds is the occurrence of a fossiliferous baud, 

 at a level of about 120 feet above the exposure of the lava. It lies 

 some 200 yards south of "Warren House, on the edge of the plateau 

 where it overlooks Gobliu Combe. The fossils include Chonetes aff. 

 comoides, Sow., Streptorhynchus erenistria. and a bisulcate Spinfer: 

 all of which points conclusively to Group C. We have not, however, 

 succeeded in finding evidence of the underlying Zaphrentis-bedB. 

 But little limestone is visible below the lava, which is nearly the 

 lowest bed exposed by denudation in the excavation of the Combe. 

 Such evidence as there is in this locality points again to the same 

 horizon as elsewhere in the district. 



(6) Near Cadbury Camp. 



Since the volcanic rocks are not here exposed in situ, we do not 

 attempt to discuss the question of their horizon. They seem, how- 

 ever, to lie in about the same position. Mr. Vaughan tells us that 

 in cuttings by the side of the track which follows the telegraph- 

 posts along the top of the ridge, that is to say, at a stratigraphical 

 level somewhat below the probable outcrop of the ' trap," the 

 Zaphrentis-beds are strongly in evidence, and he has collected most 

 of the typical fossils of Group A. 



It will be seen that in all the localities where the position can 

 be approximately determined, the evidence is sufficient to justify the 

 conclusions (1) that there is one igneous group marking a single 

 volcanic episode, and (2) that this occurred at a period which is 

 marked by the occurrence of the marine fauna indicated by Group B 

 in the table supplied by Mr. Vaughan (p. 148), to whom we desire 

 again to offer our thanks for his assistance. 



IV. The Petrology of the Igneous Rocks. 



But little has been written on this subject. In Appendix I to the 

 Survey memoir on the Geology of the East Somerset & Bristol Coal- 

 fields are descriptions by Mr. Butley, of the Uphill and AVoodspring 

 traps, and one of the ashes from "Wrington Warren. In Sir Archibald 

 Geikie's & Mr. Strahan's account, the petrology is incidentally 

 dealt with, and a description of the Spring-Cove lava by Mr. Tea 11 

 is given. These will be again referred to in due course. 



(A) The Lavas. (PI. XVII, figs. 1-3.) 



The freshest and most interesting lavas are those of Goblin Combe 

 and Spring Cove, and it will perhaps be best to describe these some- 

 what i'ullv. 



