Page:EB1911 - Volume 20.djvu/145

 stratigraphical geology, the oolite is a division of the system (q.v.). The term appears to have been first applied in this latter sense by A. J. M. Brochant de Villiers in 1803, and through the labours of W. Smith, W. D. Conybeare, W. Buckland and others, it was gradually introduced for the calcareous rocks of the British Jurassic until it came to comprehend the whole system above the Lias. Custom still sanctions its use in England, but it has been objected that the Oolitic (Jurassic) system contains many strata that are not oolitic; and since oolitic structure occurs in limestones of all ages, it is misleading to employ the word in this way.

The oolites are usually divided into: the Upper or Portland Oolite, comprising the Purbeck, Portland and Kimeridge stages; the Middle or Oxford Oolite, including the Corallian, Oxfordian and Kellaways beds; and the Lower Oolites, with the Cornbrash, Great or Bath Oolite (Bathonian), Fullonian and the Inferior Oolite (Bajocian). The Great Oolite and Inferior Oolite are treated here.

The Inferior Oolite, called by William Smith the “Under Oolite” from its occurrence beneath the Great or “Upper Oolite” in the neighbourhood of Bath, received its present name from J. Townsend in 1813. It is an extremely variable assemblage of strata. In the Cotteswold Hills it is a series of marine deposits, 264 ft. thick near Cheltenham, but within 25 m. the strata thin out to 30 ft. at Fawler in Oxfordshire. A typical section N.E. of Dursley contains the following subdivisions:—

The basal sandy series, which is closely related with the underlying Lias, is usually described as the Midford Sands (from Midford, near Bath), but it is also known locally as the Bradford, Yeovil or Cotteswold Sands. The Pea Grit series contains pisolitic limestone and coarse, iron-stained oolite and sandy limestone. The freestones are compact oolite limestones. The ragstones are fossiliferous, earthy and iron-stained oolitic limestones. The “grits” are really coarse-grained limestones or calciferous sandstones. Between Andoversford and Bourton-in-the-Water the Inferior Oolite is represented by ragstones (Ferruginous beds, Clypeus Grit, Trigonia bed, Notgrove Freestone, Gryphite Grit) and freestones (Upper Freestones and Harford Sands, Oolite Marl, Lower Freestone). Near Chipping Norton in Oxfordshire the “Chipping Norton Limestone” lies at the top of a very variable series of rocks. In Rutlandshire, Northamptonshire and Lincolnshire the following beds, in descending order, belong to the Inferior Oolite: Lincolnshire limestone (shelly, coral-bearing and oolitic), Collyweston slate. Lower Estuarine series and Northampton Sands (hard calcareous sandstones, blue and greenish ironstones and sandy limestones). The Collyweston slates are arenaceous limestones which have been used for roofing slates since the time of Henry VII.; Easton, Dene and Kirkby are important localities. The fissility of the rock is developed by exposure to frost. Similar beds are the Whittering Pendle and White Pendle or Duston slate.

The Inferior Oolite of Yorkshire differs from that of the Cotteswold district; in place of the marine limestones of the latter area there is a thick series of sands and sandstones with shales and beds of coal; these deposits are mainly estuarine with occasional marine beds. The principal subdivisions, in descending order, are: the Scarborough or Grey Limestone series, the Middle Estuarine series with their coal seams; the Millepore series and Whitwell or Cave Oolite; the Lower Estuarine series with the Eller Beck bed and Hydraulic Limestone; the Dogger and Blea Wyke beds. The last-named beds, like the Midford Sands, exhibit a passage between the Inferior Oolite and the Lias. In Skye and Raasay the Inferior Oolite is represented by sandstones.

The economic products of the Inferior Oolite include many well-known building stones, notably those of Ham Hill, Doulting, Dundry, Painswick, Cheltenham, Duston, Weldon, Ketton, Barnack, Stamford, Casterton, Clipsham, Great Ponton, Ancaster, Aislaby (Lower Estuarine series). Several of the stones are used for road metal. Iron ores have been worked in the Grey Limestone, the Eller Beck bed, the Dogger and the Northampton beds, the latter being the most important.

The Great or Bath Oolite is typically developed in the neighbourhood of Bath, and except in a modified form it does not extend beyond the counties of Wiltshire, Somersetshire, Gloucestershire and Oxfordshire. It does not reach so far as Yorkshire, unless the Upper Estuarine series of that district is its representative. The principal subdivisions of the series are:—

An exact correlation of the Great Oolite strata in the N.E. area with those of the S.W. is not possible on account of the great variability and impersistence of the beds. Current bedding is very prevalent, and minor stratigraphical breaks are common. The absence of the typical Great Oolite from the N.E. district is probably due in part to contemporaneous erosion with overstep of the succeeding formation, and in part to local changes in the sediment in the shallow waters of this epoch. This may also explain the rapid thinning-out of the Great Oolite south of Bath, where its place may be taken, to some extent, by the Bradford Clay, Forest Marble and Fullonian.

The Great Oolite is not readily divisible into palaeontological zones, but the ammonite Perisphinctes arbustigerus may be taken as the characteristic form along with Belemnites bessinus and Terebratula maxillata. Corals (Isastraea, Thamnastria) and Polyzoa (Stomatopora, Diastopora) are abundant. Hemicidaris, Cidaris, Acrosalenia, Clypeus and other urchins are common: Pentacrinus and Apiocrinus represent the Crinoids. Terebratula, Rhynchonella, Waldheimia, Crania are the prevailing brachiopods; the common pelecypods, Pecten, Ostrea, Lima, Trigonia, Modiola; Natica, Nerinea and other gasteropods are found. Perisphinctes grandes, Macrocephalites subcontractus, Oppelia discus and Nautilus dispansus are among the more common cephalopods. The remains of fish (Mesodon, Hybodus), crocodiles (Teliosaurus), dinosaurs (Cetiosaurus,