672 NORMAN L. BOWEN 



At very many places in the British Isles granophyres associated 

 with gabbro occur. Rosenbusch notes this general association.^ 

 At many of these places the granophyre is especially rich in plagioclase 

 near albite and together with the gabbro is intruded into slaty sedi- 

 ments. The Carrock FelP granophyre and the Buttermere and 

 Ennerdale granophyre are perhaps the best-described examples. In 

 the latter case it has been demonstrated that the associated Mecklin 

 Wood dolerite has reacted with the sedimentaries.^ 



Near St. David's Head, Wales, gahhroidal sills cut Arenig shales. 

 An oligoclase-rich rock has been produced near the contact by inter- 

 action with the sediment.'* Granophyric interstices have also been 

 produced near the sediment even where the contact-chilling effect is 

 noticeable. It seems impossible to deny the influence of the sediment 

 in the production of these granophyric interstices. 



It would, of course, be quite unsafe to assume that the hydrothermal 

 action here postulated was the dominant control in the development 

 of the British granophyres mentioned. Some granophyres (micropeg- 

 matites) are believed by certain authors to have been produced by 

 direct assimilation of sediments. ^ The result has been in these cases 

 essentially a normal potash-rich granite. As pointed out before, 

 assimilation of sediment has been noted in the case of some of the 

 British rocks described, and this introduces new complications. It is 

 rather their general tendency to richness in soda which indicates that 

 hydrothermal action on the sediments may have had a part in their 

 formation. 



If albite-rich rocks were normal differentiates from a certain class 

 of gabbroidal magma, they ought to be found in any association. 

 The kind of "country rock" should make no difference. A search 

 of the' literature, summarized in the foregoing, seems to indicate that 

 the kind of "country rock" does make a difference. It may be that 

 argillaceous sediments are especially susceptible to the sodic waters; 



1 Mik. Phys., II, 413. 



2 A. Marker, Q.J.G.S. (1895), 125; ihid. (1896), 320. 



3 R. H. Rastall, Q.J.G.S. (1906), 268. 



4 J. V. Elsden, Q.J.G.S. (1908), 275-76. 



s W. S. Bayley, Bull. lOQ, U.S.G.S.; R. A. Daly, "Secondary Origin of Certain 

 Granites," A. J. Sc. (4); A. P. Coleman, "The Sudbury Laccolithic Sheet," J.G., XV, 

 773- 



