Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/368

Rh 354 and Jurassic times, and containing a remarkable blending of true Palacozoic organisins with others as characteristically Mesozoic. lt is divided into two great series :—(1) Lower Trias, consisting of (a) W'crI'en Shales mid tiuttenstein Limestone, and (I2) 'irgloria Liiiie- stone or alpine Musehelkalk ; and (2) U ppcr 'l‘rias,—a. varied series of strata in three leading groups, having a united thickness soiiie- times of tlioiisands of feet. NORTH A.rERIc.i.——Rocks which are regarded as equi- valent to the European Trias cover a large area in North America. On the Atlantic coast they are found on Prince Edward's Island, New Brunswick, ant N ova Scotia, in Connecticut, New York, Pennsylvania, and N ortli Carolina. Spreading over an enormous extent of the western territories, they cross the Rocky .Iountaius into California and British Columbia. They consist mainly of red sandstones, passing sometimes into conglomerates, and often including sliales and impure limestones. On the Paciﬁc slope they contain distinctly marine organisms, which include a mingling of such Palzeozoic genera as Spirifer, Ort/Loceras, and Gozzicctites, GEOL with the characteristically Secondary genus ilmmonites. In | the centre and east of the continent they are marked by the occurrence of terrestrial plants, and in Connecticut by abundant footprints of land animals. The fossil pl-aiits present a general facies like that of the European Triassic ﬂora, among them cycads, including some of the European species of Plerop/L_i/llum. Ferns (Pecopteris, iVezu'opte7'z's, Clat/u'opteris), calamites, and conifers are the predominant forms. The fauna is remarkable chieﬂy for the number and variety of its vertebrates. The labyrinthodonts are represented by footprints, from which upwards of ﬁfty species have been described. Saurian footprints have like- wise been recognized, but in a few cases their bones also have been found. These saurians had soiue bird-like characteristics, among others that of three—toed hind feet, which producer] impressions exactly like those of birds. It is by no means certain, therefore, that what have been described as ornitliicliiiites were not really made by dinosaurs. A small iiisectivorous inarsupial (Di'o2imt/ze7'i{mz), found in the Trias of North Carolina, is the oldest American mammal yet known. J URASSIC. The next great period of geological time is termed the Jurassic, from the J ura Mountains, where the deposits of that age are well developed. It was in England, however, that they were ﬁrst studied by VVilliam Smith, in whose hands they were made to lay the foundations of strati- graphical geology. The names adopted by him for the subdivisions he traced across the country have passed into universal use, and though some of them are uncouth English provincial names, they are as familiar to the geologists of France, Switzerland, and Germany as to those of England. The Jurassic formations stretch across England in a vary- ing band from the mouth of the Tees to the coast of Dorset- shire. They consist of harder sandstones and limestones interstratiﬁed with softer clays and shales. IIence they give rise to a characteristic type of scenery,——the more dur- able beds standing out as long ridges, sometimes even with low cliffs, while the clays underlie the level spaces between. Arranged in descending order, the following subdivisions of the English Jurassic system are recognized :— Maximum lilllCkn0P§SC:. C ’. __' Upper fresh-water beds L ‘5 5 2,; Pui-beck. Middle marine beds ...................... .. 360 ‘C35 :5 Portland Stone. ............. .............  70
 * 5 2 Lower fresh-water beds ..
 * 3 L 4 Portland. Portland Sands . .............................. .. 150

Kimcridge Clay ................................ .. 600 ‘S-3 ,5 {Coralline C,. 3 ;. 3 Oomc oral hag and Calearcoiis Grit ............. .. 250 3’:"3= 0 f (1 .._. " : K 01‘ - Z O c l Clay. Oxford Clay and Ixellaways P.ocl:.. 600 O G Y [vi. STRATIGR.-KPIIICAL. Muxiniuni tliicliliesst-s, Feet. Cornhrasli, Bradford Clay ................. .. 41) .5 Great and Fui'est Marble (in l)0l‘S(‘i5lll1‘c -1.70 ft.) 3') E Oulite. Great or Bath Uolite with Stoiiesliehl) 1.,” _ slate (part of Nortlianipton Sand). f " E  Fuller's liartli group .......................... .. 150 ‘5 {('lii-lteiiliiiiii beds ............................. .. 137')
 * 5 liiferior ’.lidtord Sands (and perhaps part of’

B Oolite. 1 Nortliaiiipton Siiiids), “l)ogger” of 100 ,3 L Y orksliire ................................. .. Upper Lias ................................................... .. -lfll) Lias. %hlul‘lsi,0lle .................................................... .. 2300 Lower Lias ................................................... .. ‘.Iu0 Although these names appear in tabular order as expres- sive of what is the predominant Ul‘110l‘1l1:1lSt1Ct‘CS$lUl] of the beds, considerable diﬁ"ereiices occur when the rocks are traced across the country. Thus the Forest Marble attains a thickness of 450 feet in Dorsetshire, but dwindles down to only 15 feet at Blenheim Park. The Inferior Oolite con- sists of iiiariiieliiiiestones and sliales iii Gloncestershire, but ' chieﬂy of massive estuarine sandstones and shales in York- shire. These diﬁ"erences hclpto bring before us some of the geographical features of the British area during the J urns-ric period. The LIAS consists of three formations, well marked by physical and 1)3.liEUlliOlglCitl cliaracters. In the lower iiieiiiber numerous thin blue and brown liniestoiies with partiiigs of dark shale are surmounted by similar sliales with occasional nodular limestone bands, the whole being divided into seven zones, each cliaracterized by the presence of one or more distinctive species of amiiioiiite. 1"roiii this point of the geological series up to the close of the Mesozoic foriiiations, the ammonites play a chief part among the mollusks. The Middle Lias, consisting of argillaceous lime- stones (niarlstones) with micaceous sands and clays, is divided into ﬁve aininoiiite zones. In its Yorkshire develop- ment this subdivision is remarkable for containinga bed of earthy carbonate of iron 15 to 20 feet thick, which has been extensively worked in the Cleveland district. The upper division is composed chieﬂy of clays and shalcs with nodules of liniestones, among which three aiiiiiioiiite zones have been noted in Yorkshire. The organic remains of the Lias comprise leaves and other remains of cycads (Palceozamia), coiiifc1's(I’initcs, Cuprcssus, 1’rucc), fern.-2 (0toptcris, Alcthoptcris, &c.), and recds (Er_]m‘scti'tc.s). These fossils serve to indicate the general character of the flora, which seems now to have been mainly cyeadaccous and coniferous, and to have pl'CSL‘lltt"Il agreat contrast to the lycopodjaccous vegetation of l‘al:i-ozoie tiiiic-_-. The occurrence of laiid-plaiits dispcrsedly throughout the l'Iiigli.-li Lias shows also that the strata, though chieﬂy marine, 'L re lll'1)U>'ll1 (l within such short distaiii,-e from shore, as to l'‘('l'l'0 from time to time leaves, seeds, fruits, twigs, and stems from the lilllll. l-‘urtliti- evidence in the same direction is siippliezl by the iiiinierous insect remains, which have been obtained principally from the Lower I .ias. 'l‘licse were, no doubt, blown off the land and fell into shallow water, where they were preserved in the silt on the bottom. The .'curop- tcra arc iiunierous, and iiie.liide several species of Libcllula. Thu- colcoptcroiis forins comprise a number of beetles. There were lllu'- wise rcpresentativcs of the orthoptcrous, hemipterous, and tlipti-ro'.i.~; orders. These relics of iiisect-life are so abundant in some of the calcareous bands that the latter are known as insect-beds. With them are associated remains of terrestrial plants, eypi id.-, and mollusks, sometimes marine, sometimes apparently bi':u-ki.~li—w:iter. The marine life of the period has been abundantly pi-est-i've«l, so far at least as regards the comparatively shallow and juxta-littoral waters in which the Liassic strata were accumulated. (‘oi-ails, though on the whole scarce, abound on some horizons(Is~as!r(ca, Jlonl/z‘ralh'u, Scptastrwa, &c.). The erinoids were represented by thick growtlis of Extracrinus and I’cntrwri1Lus. There were several kinds of Stat‘- fishes, and also of sea-urcliiiis (C'i(l(Iris, llimlcma, Acrosalcm'a)—— all generically distinct from those of the Palieozoic periods. The contrast between the Liassie criistacca and those of the older systems, as Phillips has pointed out, is very dceidcd, the ancient trilobites having entirely disappeared, and having been succeeded by tribes of long-tailed ten-footed lobsters and prawns. There is a similar striking difference between the mollusks of the Lias and those of the Palaeozoic rocks, bearing witness to the great biological