Vol. 6$.~] AND CORALLIAN ROCKS OP JBRILL. 37 



III. The Arxgrove Stone (~R,haxella-Qe.vrt). 



The only well-marked Coralliau rock in the Brill district is one 

 referred to thus by Phillips : — 



' At Studley, near Oxford, Dr. Buckland detected a peculiar bed of clouded 

 grey colour, and very tough and dense texture, a sort of argillaceous chert, rich 

 in pinnae, ammonites, and other organic remains.' 1 



A. H. Green, in the Geological-Survey memoir, notes that 



? this bed runs with a good escai*pment by Arngrove Farm to the north of 



Gravel-Pit Farm, beyond which point we lose sight of it altogether 



Immediately below this stone we find Oxford Clay with Gry^hcea dilatata, and 

 it is therefore without doubt the bottom bed of the Calcareous Grit.' 2 



He gives a good list of fossils from Arngrove Farm. 



Phillips, in his ' Geology of Oxford ' 1871, makes but slight 

 allusion to this stone. He says: ' It [the Calcareous Grit] is indeed 

 actually used on the roads at Studley* (p. 298) ; and he quotes a few 

 fossils from that locality. 



Blake & Hudleston refer to it very briefly, remarking, after their 

 description of the Corallian of Wheatley : — 



' Professor Phillips has noticed, in his ' Geology of Oxford,' that the Lower 

 Calcareous Grit forms what we may call an isolated sandbank at Studley, whence 

 he has named some fossils ; but of the vast spread of this rock, marked in the 

 Survey map between Holton and Brill, not a trace can be seen. No sand- 

 pits or quarries are to be heard of ; and the whole soil indicates a stiff clay 

 beneath.' 3 



It is strange that the strongly-marked peculiarities of this rock 

 have attracted so little attention. It is a thinly -bedded stone, broken 

 up into roughly-rectangular blocks, sometimes as much as 4 inches 

 square and 1| inches thick, but often smaller. Near the surface it 

 becomes rubbly, almost gravelly in character. Its outcrop is marked 

 at intervals by patches of uncultivated, furze-covered land, on which 

 shallow pits have been opened, the stone being much used in the 

 surrounding clay-land for flooring muddy places in the fields. The 

 chief present diggings are near Arngrove Farm ; and as this is the 

 only locality from which any abundance of fossils can be obtained, 

 I have been accustomed to use the name Arngrove Stone as a 

 convenient field-term for the rock. Without intending to put it 

 forward as a permanent addition to stratigraphical nomenclature, 

 I shall use it for convenience in this paper. 



When examined with a hand-lens, this stone is seen to be studded 

 with innumerable minute ellipsoidal bodies, mostly of a translucent 

 blue, but with everywhere some of an opaque white. On some 

 surfaces these are seen to have disappeared, leaving empty spaces : 

 and sometimes, on a joint-surface, they are seen in section, when a 

 concentric structure can be detected. These bodies are embedded in 

 a matrix of a grey colour and of brittle character, weathering to a 

 light yellow-brown, and becoming very soft and friable. It is neither 



1 • Manual of Geology ' 1855, p. 307. 



2 l Geology of the Country round Banbury, &c.' (Sheet 45) 1864 p 44. 



3 Quart. Journ. Geol. Soc. vol. xxxiii (1877) p. 311. 



