R. H. Rastall — Minerals of Lower Greensand. 265 



and is followed upwards by a mass of thoroughly vesicular lava 

 disposed in rudely shaped pillows which are more or less separated 

 from one another by chert rich in carbonate. The pillows only 

 occasionally show a concentric arrangement of their vesicles. 



The carbonate of the chert- carbon ate rock just mentioned occurs 

 as lenticular aggregates, sometimes showing, under the microscope, 

 a dark centre, and also zones of growth marked by layers of dirt. 

 It looks as if the carbonate had been precipitated first, followed by 

 the chert. From analogy one may consider it possible that 

 micro-organisms have played a part in the production of the 

 deposit, though this is by no means certain. What I think is clear, 

 is that the rock is in no sense a vein-stone. Mr. Ussher and 

 Dr. Flett (op. cit., p. 83) have described a very similar chert from 

 a quarry near Devonport Workhouse on the mainland. The Drake's 

 Island chert weathers verj r readily to a soft rusty sponge through 

 the removal of its carbonate crystals. 1 



Junction D E. — Upward passage from lava to tuff very well seen 

 (though partially built over) in the cliff face of the south-west 

 shore. The dip here is about 35°. Many local faults cross the 

 junction; in fact, the whole of Drake's Island is cut up by small 

 faults. 



E. Two hundred feet of well-bedded tuffs. 



Dykes cutting B, D, mid E. — Four narrow dykes with rather 

 conspicuous felspar phenocrysts have been traced for short distances 

 through the volcanic rocks of Drake's Island and Little Drake's, 

 though only two are shown on the sketch-map. They are for the 

 most part two feet thick, and they share in the cleavage of the 

 tuffs and lava through which thev cut. 



V. — The Mineral Composition of the Lowke Greensand Strata, 

 of Eastekn England. 



By E. H. Eastall, M.A., F.G.S. 



(Concluded from p. 220.) 



6. Sandy. 



rPHE well-known section alongside the Great Northern Rail way just 

 J_ north of Sandy Station shows a total thickness of about 50 feet 

 of ferruginous sands with occasional concretionary ironstone bands,, 

 resting on the Oxford Clay. This is therefore the lowest portion of 

 the Lower Greensand. Numerous specimens from different levels in 

 the exposure have been collected and examined, but they were all 

 found to be very similar, and as a matter of fact not very interesting,, 

 so that a short description will suffice. 



1 Mr. Worth describes and illustrates additional occurrences of chert- 

 carbonate rock from Plymouth ; he regards the type as a product of the 

 contact alteration of slate (op. cit., pp. 234-6, pi. iv, figs. 20-2). Professor 

 T. J. Jehu and Dr. E. Campbell have found chert-carbonate rock among the 

 fragments of a volcanic breccia on the border of the Scottish Highlands ; " The 

 Highland Border Eocksof the Aberfoyle District," Trans. Eoy. Soc. Edinburgh, 

 vol. hi, p. 183, 1917. 



