become shallow
61Cambrian Period — Oldest time division of the Paleozoic Era. During the Cambrian, 543–490 million years ago, there were widespread seas and several scattered landmasses. The largest continent was Gondwana. The average climate was probably warmer than today, with… …
62tunnels and underground excavations — ▪ engineering Introduction Great tunnels of the world Great tunnels of the worldhorizontal underground passageway produced by excavation or occasionally by nature s action in dissolving a soluble rock, such as limestone. A vertical opening …
63Sedimentary rock — Middle Triassic marginal marine sequence of siltstones (below) and limestones (above), Virgin Formation, southwestern Utah, USA Sedimentary rock are types of rock that are formed by the deposition of material at the Earth s surface and within… …
64Atmosphere of Jupiter — Cloud pattern on Jupiter in 2000 The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System. It is mostly made of molecular hydrogen and helium in roughly solar proportions; other chemical compounds are present only in… …
65chondrichthian — /kon drik thee euhn/, n. any member of the class Chondrichthyes, comprising the cartilaginous fishes. [ < NL, equiv. to chondr CHONDR + Gk ichthýes (pl.) fish] * * * ▪ fish class Introduction class name Chondrichthyes, or Selachii any… …
66earthquake — /errth kwayk /, n. 1. a series of vibrations induced in the earth s crust by the abrupt rupture and rebound of rocks in which elastic strain has been slowly accumulating. 2. something that is severely disruptive; upheaval. [1300 50; ME erthequake …
67gastropod — /gas treuh pod /, n. 1. any mollusk of the class Gastropoda, comprising the snails, whelks, slugs, etc. adj. 2. Also, gastropodous /ga strop euh deuhs/. belonging or pertaining to the gastropods. [1820 30; < NL Gast(e)ropoda a class of mollusks.… …
68ship — shipless, adj. shiplessly, adv. /ship/, n., v., shipped, shipping. n. 1. a vessel, esp. a large oceangoing one propelled by sails or engines. 2. Naut. a. a sailing vessel square rigged on all of three or more masts, having jibs, staysails, and a… …
69soil — soil1 soilless, adj. /soyl/, n. 1. the portion of the earth s surface consisting of disintegrated rock and humus. 2. a particular kind of earth: sandy soil. 3. the ground as producing vegetation or as cultivated for its crops: fertile soil. 4. a… …
70Silurian Period — Interval of geologic time, 443–417 million years ago. The third period of the Paleozoic Era, the Silurian follows the Ordovician Period and precedes the Devonian. It marks the first appearance of land plants and jawed fishes. The continents were… …
