JURASSIC, a term used to indicate the second period of the Mesozoic era and the sys tem of rocks formed at that time. Jurassic is preceded by Triassic and followed by Creta ceous. The formations of this period take their name from their prominent exposure in the Jura Mountains; but are known in Great Britain as the (Mites because there made up of granular (oolitic) limestones below the Cretaceous and above the Lias, from the latter of which the series is separated by no very definite boundary. In portions of America, Triassic and Jurassic are not easily differentiated and the inclusive term Jura-trias is often used. No definitely recognisable Jurassic rocks are known in United States east of the Mississippi River. In the Rocky Mountain region, the area was above water throughout early Jurassic as in Triassic, but late in the period an arm of the sea en croached from the Arctic, extending as far south as Wyoming, and thin marine limestones, sandstones and shales were laid down. These contain many fossils of Arctic relationships. Before the end of the period this sea had again retreated. On the Pacific Coast, marine con ditions prevailed over the present site of the Sierra Nevada Ranges. The period was brought to a close by a great mountain-making revolu tion that upheaved the Sierra Nevadas and Cas cades. At the same time great masses of granite
(batholiths) were intruded.
Life of the During the Jurassic the most notable development of invertebrate life was among the Ammonite Cephalopods, which differentiated rapidly, and hence formed valu able time markers. Among vertebrates, the rep tiles are easily dominant. The dinosaurs de veloped along several lines. Bipedal forms were common and were of two groups, the carniv orous and the herbivorous. Quadrupedal forms grew to be of enormous size, brontosaurus reaching a length of 60 or 80 feet and weighing over 30 tons. Strange armored forms also de veloped with long spikes on their tails. Marine reptiles also were abundant, ichthyosaur (fish reptile) having a truly fish-like tail and fins. Flying reptiles, known as pterosaurs (wing rep tiles) became common. The larger ones had a wing spread of 20 feet and had hollow bones as do birds. The first true bird (Arcigeopteryx) (q.v.) has been found preserved in Jurassic rocks. It had feathers like modern birds, but was furnished with long, sharp teeth. A few minute remains believed to represent mammals have been found, the earliest remains of the mammal group. The plant life was very similar to that of the Triassic (q.v.).