part 1] SPIT.ITES IN DERBYSHIRE. 19 



such as are never seen in tlie normal black chert so abundant in 

 nodules and thin layers in the upper beds of the Carboniferous 

 Limestone of Derbyshire. In the Crackendale chert-beds, too, all 

 the original organisms of the lime«^tone appear to have been 

 entirely destroyed, although in the limestone above and below the 

 chert fossils are abundant. The chert-beds at the Holme-Bank 

 Mine have an average thickness of 4 feet, and the rock is white 

 or grey in colour : it is mined for use in the Potteries. 



IV. Petrogenetic Considerations. 



The form of alteration to which the Derbyshire spilites have 

 been subjected obviously is quite different from that which results 

 from ordinary weathering, and appears to be clearly in the nature 

 of autometamorphism. All the available field-evidence that I 

 have hitherto noted tends to show that the spilitic rocks generally 

 underlie (in the same bed) those of more basaltic type, and it is 

 sometimes possible to observe a progressive upward variation. This 

 has already been noted in the description of the lavas on Knot Low 

 and in Tideswell Dale. 



These observations agree with those recorded by other investi- 

 gators, from Charles Darwin ^ (more than seventy years ago) to 

 Prof. P. Marshall ^ in his paper read before the Geological Society 

 in May 1914, to the effect that, where alkaline and calcic lavas 

 proceed from the same orifice, the former have generally been 

 erupted first. 



It is not claimed that the Derbj^shire spilites are of extreme 

 alkaline type; but it is suggested that, in considering their genetic 

 relations to the associated basalts, we may premise a magma of 

 which the upper part w^as richer in gases and in the more easily 

 sublimable constituents, including the alkalies, than the remainder, 

 and that, as this upper portion was drawn off, the later erupted 

 material would tend to become increasingly calcic. The constantly 

 vesicular nature of the spilites is sufficient proof of the presence of 

 a large amount of gases and residual water. 



That no general stratification of the magma, due to the action 

 of gravity, took place prior to eruption is, however, clear from the 

 fact that the earliest flows, the spilites, always contain a notable 

 proportion of the first-formed minerals — olivine and iron-ores. 



Without entering on the vexed question of the origin of alkaline 

 rocks in general, on w^hich so much has been written of late years, 

 I may point out that the relative enrichment in alkaline con- 

 stituents of the upper part of the magma, postulated above, is 

 probably assignable to the upward passage through the magma 

 of gases carrying Avith them a considerable portion of the alkalies 



^ ' Geological Observations on the Volcanic Islands ' 1844, p. 118. 

 2 Q. J. G. S. vol. Ixx (1914) pp. 382-406. 



c2 • 



