part 1] SPILITES IX DEEBTSHIEE. 21 



mineral, since there can be little doubt that the hydrated residue 

 'Contained all the elements- necessary to its formation. The occm*- 

 rence of amygdaloidal albite is somewhat rare : in the Tideswell- 

 Dale rock, when it occurs in the vesicles, this albite forms the 

 .entire infilling, with the exception of the thin chloritic lining. 



The other infilling minerals are all clearly of secondary origin, 

 :resalting from the breakdown of the ferromagnesian constituents. 

 They are, moreover, all minerals that may be formed at fairly high 

 temperatures, and it is noteworthy that no hydrated minerals that 

 .are formed at lower temperatures than albite, such as zeolites of 

 various species, and 2)rehnite, are present (so far as my observation 

 goes) in any of the Derbyshire lavas, whether spilites or basalts. 

 The synthesis of both albite and orthoclase has been effected by 

 various investigators at about 500° C.^ 



Mr. W. F. P. McLintock 2 has" shown that in the vesicles of 

 Beinn Fhada (Mull) the order of deposition, apart from chlorite, 

 which there, too, may be formed at any stage, was : albite, epidote, 

 prehnite, scolecite. These vesicles were evidently fi^lled under 

 conditions of falling temperature ; for, with rising temperature, 

 •occasioned by an intrusion of granophyre, the metamorphic changes 

 in lime-bearing silicates resulted in the production of minerals in 

 the reverse order: scolecite, prehnite, epidote, garnet. Since all 

 the elements necessary to the formation of minerals of the zeolitic 

 class so frequently found in basalts were undoubtedly present in 

 the residual solutions of the Derbyshire lavas, either as primary 

 •constituents thereof or as alteration-products of the earlier-formed 

 minerals, the inference seems to be justified that the consolidation 

 •and the autometamorphism of the Derbyshire spilites were botli 

 •completed before the temperature fell much below 500° C. 



V. Analyses. 



Analyses of two of the spilitic lavas of Derbyshire are given 

 •on p. 22 (Nos. IV & V) and partial analyses of a third (Nos. VI & 

 VII). For comparison, analyses of a mugearite and two spilites 

 iire also quoted. Such intensely altered rocks as these cannot, of 

 course, be expected to show the composition of the original rock. 

 Nevertheless, it is clear that we are dealing here with essentially 

 basic rocks rich in alkalies, especially potash, but much altered 

 with production of carbonates and hydrated secondary material. 

 If we assume the carbon dioxide to be combined with lime in the 

 form of calcite, an assumption which is borne out by the micro- 

 scope, it will be seen that there is very little lime left for lime- 

 felspar. 



^ See C. Doelter, ' Minerogenese & Stabilitatsfelder der Minerale ' Tscher- 

 mak's Min. Petr. Mittheil. vol. xxv (1906) p. 103; also F. W. Clarke, 'The 

 Data of Geochemistry ' Bull. U.S. Geol. Surv. No. 616, 3rd ed. (1916) p. 366. 



^ ' On the Zeolites & Associated Minerals from the Tertiary Lavas around 

 Ben More (Mull)' Trans. Roy. Soc. Edin. vol. li (1915) pp. 24-30. 



