On a Spilitic Facies of Lower Carboniferous Lava-Flows in Derbyshire

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Abstract
I. Introduction. The igneous rocks of Derbyshire, locally called ‘toadstones,’ 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 limestone. 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 prolonged subsidence. Dr. Bemrose 1 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 Geologists' Association to Derbyshire in the summer of 1914, specimens of lava were collected in Tideswell Dale which appeared, on examination under the microscope, 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 affinities with the spilites of Devon and Cornwall, and also, to some extent, with the mugearites of East Lothian. It seemed desirable, therefore, to make a further examination of the rocks of the district, in

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