part 1] SPILITES IN DEEBYSHIEE. 17 



trachjtic and they are rich in alkalies. They are, however, dif- 

 ferentiated from the spilites of Cornwall by their abundant content 

 of olivine and the presence of orthoclase. On the other hand, these 

 features are characteristic of the niugearites. 



Pillow- structure, again, has not been recorded among the Derby- 

 shire lavas, but this feature is probably dependent on accidental or 

 local chcumstances. Mr. C. lieid & Mr. H. Dewey i have suggested 

 that it may, perhaps, result from some special physical capacit}- for 

 retaining bubbles of gas, and be quite independent of chemical 

 composition. 



It is clear therefore, in view of the characteristic features of the 

 Derbyshire lavas described above, that, while they are intermediate 

 geographicall}'- between the spilites of the South- West of England 

 and the mugearites of the Lothians, they are also intermediate in 

 character. Their structure and field-relations appear to me to asso- 

 ciate them, on the whole, more closely with the former than A\ith 

 the latter, and, in view of their outstanding peculiarity, it is sug- 

 gested that they may be ajDpropriately termed potash-spilites. 



It is interesting to note the striking resemblance between the 

 spilitic lavas of Derbyshire and some of the rocks of similar age 

 associated with the Carboniferous Limestone of the Bristol district.^ 

 which appear to be also of sj^ilitic type. The preponderance of 

 potash over soda is as marked there as in the Derbyshire lavas. 

 The average of five analyses of the Bristol rocks shows the follow- 

 ing percentages: — potash = 5'028, soda=0-854.3 There, too, as in 

 Derbyshire, the rocks of spilitic type are associated with true 

 basalts. 



III. The Associated Rocks. 



Just as the mugearites of Skye are associated with the Roineval 

 dolerites,* those of the Lothians with olivine-basalts,^ and the 

 spilites of Devon and Cornwall Avith albite-diabases,^ so, too, in 

 Derbyshire, magmatic differentiation has produced similar results 

 which are seen in the association of the spilites of that county with 

 olivine-basalts. These have labradorite or a more basic species for 

 their dominant felspar; but, for the reason already noted, there is 

 no object in describing them here in detail. 



The close association of spilites with jaspers and cherts, the 



' Q. J. G. S. vol. Ixiv (1908) p. 268. 



- S. H. Reynolds, ' Further Work on the Igneous E-ocks associated with 

 the Carboniferous Limestone of the Bristol District ' Q. J. G. S. vol. Ixxii 

 (1916-17) pp. 23-41. 



=* Ihid. p. 38. 



"* ' The Tertiary Igneous Eocks of Skye ' Mem. Geol. Surv. Scotland, 1904, 

 pp. 262-64. 



^ • The Geology of East Lothian' Ihid. 1910, pp. 122-23. 



'' H. Dewey & J. S. Flett, ' On some British Pillow-Lavas & the E.ock& 

 associated with them ' Geol. Mag. dec. 5, vol. viii (1911) p. 206. 



Q. J. G. S. No. 289. c 



