parti] LOWER CARBO^^IFEllOUS SPILITES, 11 



2. On a Spilitic Facies o/'Lowee Cakeonieerous Laya-Flows 

 in Deeey'shtee. By Henjiy Ceuxdex Saegext, F.Gt.S. 

 (Read February 7th, 1917.) 



Contents. 



Page 



I. Introduction , 11 



XL The Lavas 12 



III. The Associated Rocks 17 



IV. Petrogenetic Considerations 19 



V. Analyses 21 



VI. TheTuffs 22 



VII. Summary of Conclusions 23 



1. Inteoductiojt. 



The igneous rocks o£ Derbyshire, locally called ' toadstoiies,' form 

 a well-known basic series comprising lava-flows, sills, a few thin 

 dykes, and also, in many localities, pyroclastic material varying 

 from coarse agglomerate to fine-grained tuff. All these rocks occur 

 entirely in beds of Lower Carboniferous age. The lava-flows and 

 ashes were contemporaneous with the Carboniferous Limestone, 

 with the exception of a few small occurrences in the south-west of 

 the county which are in the Limestone Shales overlying the lime- 

 stone. That they were submarine is evident from their intercalation 

 at various horizons in marine deposits. There would appear to have 

 been a large number of vents scattered over a sea-floor that was 

 undergoing slow and ^^rolonged subsidence. 



Dr. Bemrose ^ has classified the lavas and sills under three heads, 

 namely: (1) olivine-dolerites, (2) ophitic olivine-dolerites, and (3) 

 olivine-basalts. These distinctions rest on internal structure alone, 

 and not on mode of occurrence, intrusive or extrusive. 



In the course of a visit of the Greologists' Association to Derb}^- 

 shire in the summer of 1914, specimens of lava were collected in 

 Tideswell Dale which appeared, on examination under the micro- 

 scope, to vary widely in character from the dolerites and basalts of 

 the county hitherto described. Dr. Harker, to whom a thin section 

 was submitted, pointed out that it possessed decided aftinities with 

 the spilites of Devon and Cornwall, and also, to some extent, Avith 

 the mugearites of East Lothian. It seemed desirable, therefore, 

 to make a f urthei- examination of the rocks of the district, in order 

 to ascertain to how great an extent these features occurred , else- 

 where, and the object of this paper is to record the result. 



I desire to express my great indebtedness to Mr. A. T. Metcalfe, 

 F.G-.S., who was the first to note the special features of the 

 Tideswell-Dale rock and urged me to undertake the investigation. 



1 Q. J. G. S. vol. 1 (1894) p. 624. 



