part 1] SPILITES I>r DEEBTSHIEE. 23 



VII. Summary of Concltjsions. 



(1) The lavas of Derbyshire, which are all of Lower Carboniferous 

 age, embrace rocks of spilitic or iiiugearitic type and basalts. Both 

 types have been derived from a common magma. The field-relations 

 hitherto ascertained indicate that, when they occur in association, 

 the spilitic rocks always underlie, and may grade upwards into, the 

 basalts. 



(2) The differentiation was effected by the partial concentration, 

 prior to eruption, of the alkaline constituents in the upper part of 

 the magma, whither they were probably carried in a volatile state 

 by the upward movement of gases in the magma. 



(3) The intense alteration which the spilitic rocks have under- 

 gone was effected shortly after extrusion by hot residual solutions 

 belonging to the lava itself, and not by post-volcanic emanations 

 or weathering agencies. 



(4) The deposits of quartz-rock and chert, which frequently occur 

 in Derbyshire in the neighbourhood of vents, are probably the result 

 of volcanic emanations of which it is difficult to define the precise 

 composition. There appears to be no doubt that they contained 

 silicates and carbon dioxide in solution, and other substances may 

 have been present. Silica liberated from the lavas as a result of 

 pneumatolytic alteration may also have contributed to the same 

 result. 



Discussion. 



The Peesideis^t (Dr. A, Haeker) welcomed this contribution to 

 the petrology of the Carboniferous lavas. The interesting alkaline 

 types described had been hitherto neglected, owing parth^ to a ten- 

 dency to select for study the fresher-looking material. It would 

 appear that in Lower Carboniferous times the British area included 

 two petro graphical provinces, both characterized by rock-types rich 

 in sodic felspars, but having different histories. The southern or 

 Cornish province was an old-established one, and was spilitic : the 

 Somerset lavas must be included here. The northern province 

 represented a reaction from the very different Caledonian regime, 

 and was marked by the prominence of olivine-basalts and muge- 

 arites, Avith some soda-trachytes : here belong the Scottish and 

 Irish districts with the Isle of Man. In Derbyshire, occupying 

 geographically an intermediate situation, the volcanic rocks seem 

 to be mainly of Scottish types, but with spilitic affinities indicated 

 in some of the occurrences. 



Mr. E. B. Bailey queried the application of the term ' spilitic ' 

 to a suite of rocks in which ])seudomorphs after olivine are com- 

 monly found. He thought that the decomposed lavas of Derby- 

 shire might be compai-ed perhaps with albitized basalts and 

 mugearites from the Scottish Carboniferous, rather than with 

 the numerous occurrences of spilites described by Dr. Flett from 



