OOLITE or .ItinAssrc GROUP (in Geology). an extensive and important series of strata of secondary age, underlying the chalk formation, and resting on the trias. In Britain they received the name Oolite because in the district where they were first examined and described by Dr. W. Smith, the limestones contained in them had an oolitic structure (see foregoing article). 'The name Jurassic has been given to them on the continent because the range of the Jura mountains in the n.w. of Switzerland is almost entirely composed of them. The strata of the group have been arranged in the following order. The maximum thickness of each division is given in feet: It is apparent from this table that the oolitic rocks consist of three extensive clay deposits, each of which forms the basis of a smaller and variable set of sands and lime stones; the upper oolites resting on the Kimmeridge clay, the coral rag on the oxford clay, and the lower oolite on the lias.
1. The Purbeck beds, unlike the other oolitic rocks, are chiefly freshwater deposits. Though lithologieally they are very similar throughout, the of the con tained fossils have caused them to be grouped into three series—upper, middle, and lower. The upper Purheeks are purely fresh-water, containing beds of lime stone and shale, which abound in shells of lake and river mollusea and eypridus. The stone called Purbeck marble, formerly so extensively used in the ornamental architecture of English churches and other buildings, belongs to this division; it consists of the shells of Paludinm, held together by a somewhat argillaceous paste. The middle Purbecks are partly fresh-water, and partly brackish or marine. The " cinder-bed," composed of a vast accumulation of shells of Ostrea distorta, occurs in this section, and near it is the narrow layer from which Mr. Beckles recently obtained the remains of several mammalia. The lower Purbecks are chiefly fresh-water, with some intercalated brackish or marine beds, and one or two old vegetable soils called by the quarrymen " dirt-beds," which contain the stems of eyeadaceous and coniferous plants. 2. The Portland beds consist of oolitic and other limestones interstratified with clays, and passing below into sands and sandstones, from which the well-known build ing-stone is obtained, of which St. Paul's and many of the principal buildings in Lon
don are built. 3. The Kimmeridge clay is generally a dark-gray bituminous shale, with intercalated beds of sand, calcareous grit, and layers of septaria. The dark shale in some places passes into an impure brown shaly coal. 4. The coral rag contains, as its name implies, an abundance of corals, in bluish limestone beds mixed with layers of calcareous grit.• The Solenhofen lithographic stone, with its beautifully preserved and varied fos sil remains, belongs to this division. 5. The Oxford clay is a dark-blue or blackish clay without corals, but having a large number of beautifully preserved Ammonites and Bcl emnites. Beds of calcareous sandstone, called Kelloway rock, occur in its lower portion. 6. The eornbrash consists of thin beds of cream-colored limestone, with sandstones and clays, and the forest marble (so named from Wyeii wood forest) is composed of an argil laceous limestone, with numerous marine fossils, blue marls and shales, and yellow offi cious sand. At Bradford, 'Wiltshire. the forest marble is replaced by a considerable thickness of blue unctuous clay. 7. The great oolite is composed of shelly limestones, sandstones, and shelly calcareous sandstones, and'the Stonesfleld slate is a slightly oolitic shelly limestone, which splits into very thin slabs, erroneously called " slates; " it Li remarkable for the remains of terrestrial reptiles and mammals found in it. The Bath oolite, a celebrated building-stone, belongs to this division. 8. The fuller's-earth group is a local deposit found near Bath; it consists of a series of blue and yellow shales and marls. some of which have properties fitting them for the use of the fuller. 9. The inferior oolite is composed of a series of beds of pisolitic and shellv limestones, brown marl, and brown sandy limestone, all abounding in fossils. 10. The has (q.v.) is a great clay deposit. It is divided into the upper and lower lies, which consist of thin beds of limestone scattered through a great thickness of blue clay, and, separating these two groups, the marlstone, or calcareous or ferruginous sandstone. The lies abounds in beautifully preserved fossils.