JURAS'SIC SYSTEM. A division of the column following the Triassie and im ?ediately preced;lig the Cretaceous. 'file name is from the Jura Mountains of Switzerland, where there is a great development of the rocks of this System. A fullness of detail is observable in ether parts of Europe; but in America the Jorassio strata are of small extent. and in places it is impossible to separate them from the Trias sic. for which reason the term Jura-TH:1s is em ployed on the maps of the Slates Con logical Survey. The main subdivisions of the Jurassic system are as follows: (a) Lias, nr Louver .Tura; (1) Lower Oidite, or Nliddle .Tura; te) Oiilite; and (t1) Upper Wilke. or Upper Jura. Strata of undoubted Jurassic age are not known along the Atlantic coast of the 1 nited States, although song geohigists have con sidered that the upper beds of the Trias are referable to this system, while other authorities would class the Potomac beds as Jurassic'. A great area of prohalde .luressie sandstone, but lacking fossils, was deposited in an interior sea in Colorado, Wyoming. Arizona, and New :Mex ico. In California and Oregon there arc Lias,ic beds, while marine Upper .Jurassic strata ocenr iu northern Utah, 'Wyoming, and Almitana. Upper Jurassie slates of great thickness and interbedded with voleanie tolls are found in the Sierra Ne vada of California and ill British Columbia. ln Europe there is an ablIlldalleP of rocks, which were deposited in depressions of post-Trias sic time. Those of the I.ias or Lower Jura cover large Southern and Central Europe, and also extend in a band across ti:reat Britain. 'They are lacking. however. in In the Lower oolite much additional land was sub merged. and the deposits cover Central and Northern Russia, Siberia, and the Indian Penin sula.
The 'Jurassic rocks abound in fossils in some areas, notably Europe, where ill England alone over 4000 have been found. The plant life of the .Jurassic is similar to that of the Triassic. Antion• the more important forms were ferns, equiseta, eye-ads. and outliers. It was in this era that the eyeads attained their maximum development, and tree-ferns grew in great pro fusion. Foraminifera are found ill countless numbers in some of the limestones, as wore also the siliceous cases of radiolarians. and sponges. Corals were and sea-urehins and eri molds swarmed. There were delicate forms of life smell as ernstaceanss limuloids. and insects of several orders required spevial condi 1•_ s for their perfect preservation. These are found in abundance in the homogeneous fine grained lithographic litin‘stones of Solenhofen, Bavaria. Brachiopods still existed in the Juras
sic, and lamellihranchs of the oyster type were very common. The cephalopods were another class which etilminated in this era, and included both imutiloids and arnmonoids among the coiled forms, and belemmites ill the straight shells. Among the fishes there was an advance over those of Triassie tittles. In the class of teleostomes. the ganoids Continued to predominate, and many were covered with thick shining scales. Ampliihia are known to have existed. lint the reptiles were a prominent feature of the Jurassic farina. Among them were turtles, lizards (the first true ones known), and ielithyosaurians, or marine reptiles, the European representative being Tchthyosatirus (q.v.). and the .American one Baptanodou (q.v.). Another marine group was represented by Ile sio.altrus, which differed from Ielithynsaurns in having a nitteh longer body and neck, and larger paddles. The assumed prodigious proportions. but were of variable shape and size. limy included the genera :tregalosaurus. Cetin saurns, Osinnsaurus. and Compsng,nathus. The Plerosaltria were flying reptiles. having a spread of wings of about three feet. They are found in the S'olenlinfen together with a more mini ons fossil, the arelveopteryx (q.v.). The latter tepresents the earliest bird known.
The Jurassic was a time of great geographical change in North America. During this era the Appalachians were subjected to extensive erosion. A gulf spread northward from the Southern United States over the great basin region, and a similar sea existed in Canada east of the Cordil leras. At the close of the -Jurassic there was a period of mountain-making along what is now the Pacific coast. The Sierra Nevadas were uplifted, and probably also the coast ranges.
The economic products of the Jurassic are few in the United States. The most important are the gold-bearing veins found in the Jurassic slates of California. and known collectively as the 'mother lode.' Beds of fire-clay and potters' clay are also found. Practically the entire sup ply of limestone used for lithographic work is obtained from Jurassic deposits near Solenhofen, in Bavaria. This is a limestone of remarkably fine grain and extremely even texture, whose equal has thus far been found at hut very few localities. Consult: Ceikie. Text-book of (;ealogy (London, 1893) ; White. "On the Fresh Water Invertebrates of the North American Jurassic," United ,Stotts Geologiral Surrey, Bulletin No. 29 (Washington, 1836). See GEOLOGY; LIASSIC ERIES GOLITE.