JURASSIC SYSTEM, in geology, the succession of rocks deposited during the middle period of the Mesozoic era, succeed ing the Trias and preceding the Cretaceous period. This great series of fossiliferous rocks was first studied in Britain by William Smith, who, in his two works Strata Identified (I816) and Strati graphical System of Organized Fossils (1817) made it the founda tion of stratigraphical geology. Many of his original names for the subdivisions in England have been preserved in the universal terminology of geology. The upper part of the system was orig inally called the Oolitic series by Conybeare and Phillips (1822) from the characteristic oolitic or roe-like structure of many of the limestones, while the underlying Lias was regarded as a separate series. The name "Jura limestones" for rocks equivalent to the Oolitic series, well developed in the Jura mountains of Switzer land, was first employed by A. Brongniart in 1829 and ten years later was adopted for the whole system by von Buch, who divided it into Lower or Black, Middle or Brown, and Upper or White Jura. The Jurassic rocks and their fossils were studied on the Continent by A. d'Orbigny, F. A. Quenstedt, A. Oppel, M. Neu mayr, P. de Loriol, H. Salfeld, and by many other distinguished workers. In England the names of John Phillips, John Lycett, J. Morris, J. F. Blake, W. H. Hudleston, H. B. Woodward, S. S. Buckman, L. Richardson, W. D. Lang, and J. Pringle stand out as the principal geologists to carry on the work begun by William Smith.
Quenstedt in Der Jura (1858) subdivided the system into 18 stages, named a to after the letters of the Greek alphabet, and based largely on the lithological divisions in the Jura mountains. Owing to the great petrological diversity of the rocks resulting from their shallow-water origin—clays, sands and limestones succeed one another in different order in different places—Quen stedt's stages have been abandoned and a palaeontological system of zoning has been introduced. The fossils most used have been ammonites, as they have been found to have the most restricted vertical range in the rocks and to be to a certain extent independent of facies. But while there has been general agreement as to the order in which the zonal forms succeed one another, the lines of division between the formations have varied with the opinions of the authors, as have also the number of zones considered worthy of recognition. In addition to the palaeontological zones, a num
ber of stages have been established, named after towns and local ities where particular groups of zones are especially well de veloped, or after any other noteworthy characteristics (e.g., Oxfordian ; Virgulian after Exogyra virgula; Fullonian after Fuller's Earth).
The Jurassic period was marked by a great extension of the sea, which commenced after the close of the Trias and reached its maximum during the Callovian and Oxfordian stages, the rocks of which consequently have the widest distribution. The marine advance was, however, checked temporarily in certain localities at comparatively early stages (as during the Bajocian and Bathonian in the north of England), and towards the close of the period (Purbeckian) fresh-water marshes became wide spread over the continental surfaces, foreshadowing the emer gence of the Lower Cretaceous land. In America the Sierras, Cascade mountains, Klamath mountains and Humboldt range be gan to emerge, but there appears to have been very little crustal disturbance or volcanic activity. Tuffs are known in Argentina and California, and certain volcanic rocks in the islands of Skye and Mull. In the Alpine region marine conditions steadily per sisted, forming the local Tithonian stage.