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IGNEOUS ROCKS OF THE BRISTOL DISTRICT. 



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VI. Summary and Conclusions. 



The igneous rocks associated with the Carboniferous Limestone 

 of the Bristol district may he arranged in three groups : — 



(a) Normal olivine-hasalts (Goblin Combe, eastern and western 

 exposures, and 97 from Furze Close, Milton Hill). All 

 these have a very low percentage of potash. 



(b) Oli vine-bearing rocks, with a rather high percentage of 

 potash and a low percentage of soda (Limeridge Wood, 

 Tickenham, and 98 from north-east of Milton-Hill Lodge ; 

 probably the rocks from Uphill and from Middle Hope or 

 Woodspring belong here). 



(c) Olivine-orthoclase basalts, with sometimes an imperfectly 

 variolitic structure, a very high percentage of potash, and a 

 low percentage of soda (Spring Cove and cross - roads, 

 Milton Hill). 



I am greatly indebted to Dr. J. S. Flett and Dr. H. H. Thomas 

 for help and advice ; also to the latter for the loan of sections 

 of rocks from the Exeter district. Sincere thanks are also tendered 

 to the following, for permission accorded to dig trial-holes : — 

 Mr. J. Gibson, J.P. (Goblin Combe) ; Mr. H. B. Napier, of the 

 Ashton-Court Estate (Limeridge Wood, Tickenham) ; and the 

 Great Western Railway Company (Uphill). 



Discission. 



Dr. A. St rajk an commented on the difficulty which had been 

 experienced in tracing these igneous rocks, owing to lack of ex- 

 posures. Much of the outcrop in Goblin Combe, as shown on the 

 geological map, was founded on the occurrence of igneous debris 

 in the soil, and in part only on actual exposures. He was not 

 satisfied that its continuity had been disproved. That the lavas in 

 the various occvxrrences in Somerset were discontinuous was obvious, 

 hut that other evidences of igneous action were confined to such 

 limited areas as the Author claimed, seemed improbable. Geologists 

 were greatly indebted to the Author for the additions that he had 

 made to their knowledge of Somerset geology by opening trial- 

 holes in doubtful ground. 



Prof. T. Franklin Siblt enquired whether the Author's further 

 investigations on Milton Hill had enabled him to utilize the 

 volcanic horizon of Spring Cove for mapping the dip-faults, later 

 in date than the great longitudinal overthrust fault which tra- 

 versed the Weston- Worle ridge of Carboniferous Limestone. The 

 faulted structure of this ridge was a typical illustration of the 

 displacements which had affected the Carboniferous rocks of 

 neighbouring areas. 



Dr. A. P. Young said that the Author's discovery of pillow- 

 lavas and associated rocks would furnish a key to many phenomena 

 observed in this area. The speaker had collected from the shore 



