OOLITIC. 283 



water. Portions of the Great Oolite and the Forest Marble are 

 conspicuously false-bedded. The formation of oolite has been thus 

 explained by INIr. Hudleston, by reference to modern Coral-reefs : — 



" The coral is being perpetually ground down to the finest powder, which is 

 held suspended in the sea like ordinary sediment ; but as it falls towards the 

 bottom, it encounters an acid stratum of water, due to the quantity of carbonic 

 acid generated by the decomposition of organic matter and the respiration of 

 animals. This slightly attacks the calcareous sediment, and forms the usual 

 soluble bicarbonate, which is again precipitated as calcic carbonate amongst the 

 interspaces of the slowly settling, mud, thus cementing the whole into a mass of 

 most compact rock, and gluing up all the shells." "From this type of rock, to a 

 regular oolite with very little paste, there is every gradation."^ 



Oolitic rocks, although characteristic of the series, by no means 

 constitute its bulk : they are interstratified with sands and clays, 

 and seemingly in places in so regular a sequence of sand, clay, and 

 limestone, that much remark has been made upon the subject.^ 

 But this repetition of a ternary or tripartite series is after all very 

 local and unreal ; the divisions that appear in the south of 

 England do not correspond lithologically with those in the 

 north of Oxfordshire and the southern part of Northamptonshire, 

 nor with those made in the northern part of Northamptonshire 

 and Lincolnshire ; whilst a comparison between the ' Yorkshire 

 Oolites ' and those of the Cotteswold Hills shows more strikingly 

 the changes undergone by the strata, for in Yorkshire the beds are 

 essentially arenaceous. It must, however, be borne in mind that 

 the deposits presented to our view are but local sediments. 



Prof. Hull has put forward reasons for concluding that all the 

 Lower Jurassic formations exposed in the western and central 

 counties die out towards the south-east ; that this attenuation is due 

 to the increase of distance from the sources of supply, and the 

 consequent failure of sedimentary materials which were derived 

 from land occupying the region of the North Atlantic.^ Such 

 attenuation may in some cases, as pointed out by Mr. Topley and 

 Mr. Lucy, modify our estimates of the dip.* (See Fig. 43.) 



As before observed, the character of the different subdivisions of 

 the Oolitic strata varies much ; the development of the oolites and 

 sandy strata being more restricted than that of the clays, for the 

 Inferior Oolite, the Great Oolite, and the Corallian Beds, die out in 

 places or are represented by strata of a different sedimentary 

 character. The Cornbrash may be cited as being the most persistent 

 of the calcareous strata. Fissile sandy limestones, locally known 

 as ' slates,' occur at various horizons in the Lower Oolitic series. 



The Oolitic strata rest conformably upon the Liassic sediments, 

 and while the entire Jurassic series exhibits many variations, the 

 absence of individual members in certain places may be due to 



^ Proc. Geol. Assoc, v. 431. 



2 E. Hull, G. Mag. 1868, p. 143 ; Q. J. xviii. 133; Phillips, Geol. O.xford, p. 393 ; 

 Lyell, Student's Elements, 1871, p. 342. 



^ Q. J. xvi. 63 ; see also Physical History of the British Isles. 

 * Q. J. XXX. 186; and Proc. Cottesw. Club, 1869, p. 7. 



