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OOLITIC, OR JURASSIC LIMESTONES. 205 these strata are nobly developed in the picturesque range of the Jura, lying on the borders of France and Switzerland, geologists both of Britain and the Con tinent are now inclined to adopt the territorial name, ‘ Jurassic/ in preference to the petrological term, ‘ Oolitic,’ when speaking of this series of strata. Structure and Specicdities. On examining a speci men of Bath or Cheltenham Oolite, we find that the spherular grains are either hollow, or contain as a nucleus a grain of sand, or a fragment of a shell, or other foreign substance. The size of other spherules is about that of the roe of a small fish; but in a few instances they are much larger. 1 Fragments, or whole shells, or skeletons of molluscs, crinoids, and corals are enclosed ; and, not unfrequently, the strata present in a conspicuous degree the phenomena of oblique-lamination, arising from the action of cur rents in the waters in which they were deposited. All these Oolitic limestones are of marine origin. They are composed of carbonate of lime, with various proportions of carbonate of magnesia, silica, alumina, and iron. When used for buildings not subjected to the smoke of cities and manufacturing towns, they often last for lengthened periods. But different beds have very different powers of resisting 1 As in the case of the pisolite, at the base of the inferior oolite at Cheltenham. See ‘ Geology of the Country around Cheltenham,’ Mem. Geol. Survey, p. 32 (1837).
