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IGNEOUS BOOKS OF THE BRISTOL DISTRICT. 



39 



The estimates marked with an asterisk in the foregoing table (2) 

 were determined in the Chemical Laboratory of the University of 

 Bristol by Mr. J. H. Sturgess, the remainder by Mr. E. Gr. Badley. 

 I am much indebted to my colleague, Lieut. D. E. Innes, B.A., 

 and to my pupil, 2nd Lieut. F. S. Wallis, for determining the 

 specific gravities. 



V. Comparison with Kocks of othee Areas. 



Chemical and mineralogical examination show the presence in 

 certain of these rocks of acid felspar : orthoclase (Limeridge Wood 

 Tickenham, and Milton Hill) and probably oligoclase (Uphill) ; 

 while, in regard to their general aspect and composition, and 

 particularly in resjject of the low silica-percentage and abundance 

 of olivine, the rocks are basalts. 



Rocks showing a similar association of olivine and orthoclase 

 have been described from several other localities. The following 

 are the most important : — 



1. Some of the Permian lavas of the Exeter district 1 are well 

 known to contain orthoclase in association with olivine ; and. though 

 in the field, owing to their generally pale and reddish tints, the 

 difference between these rocks and those of the Bristol area is 

 considerable, the resemblance both in microscopical structure and in 

 chemical composition is quite close. This is particularly the case 

 with the lavas described by Sir Jethro Teall as the Knowle-Hill 

 (Crediton) type. These rocks are, he remarks, 2 'basic so far as 

 silica percentage is concerned, but (which) differ from ordinary 

 basic rocks in containing a large amount of potash.' An analysis 

 of the rock from Knowle Quarry, Spencecombe, is quoted above 

 (p. 38), and it is evident how close is the correspondence between 

 this rock and that from Spring Cove in the percentages of silica, 

 alumina, lime, and alkalies. A section of the rock from Spence- 

 combe Farmhouse Quarry (E 3219 in the Geological Survey 

 collection) resembles very closely some of the Milton-Hill rocks. 

 Sir Jethro Teall clearly regarded the orthoclase in these rocks as 

 original, but remarks 3 on the reduction of the lime through the 

 leaching-out of the carbonate. 



2. Dr. Flett suggested comparison with certain of the rocks of 

 Pelvoux described by Prof. Termier, 4 particularly with certain 

 ' diabases ' which, while originally rich in lime, now contain much 

 potash. The changes in these rocks that are entirely attributed 

 to the metasomatic action of surface-water, are twofold : (<?) the 

 elimination of the bulk of the lime, and to a less extent of the 

 alumina and magnesia ; (Z>) the addition of potash, which is 

 believed to be derived from the neighbouring mass of the Pelvoux 



1 See ' Geology of the Country around Exeter' (sheet 325), Mem. G-eol. 

 Surv. 1902, pp. 55-85. 



2 Ibid. p. 83. 3 Ibid. p. 79. 



4 Bull. Soc. Geol. France, ser. 3, vol. xxvi (1898) pp. 165-92. 



