GEOID 



Geoid. Term invented to de- 

 scribe the shape of the earth. Like 

 theother planets, the earth is spheri- 

 cal, but not a true sphere. The 

 bulge of the earth which makes its 

 equatorial diameter longer than 

 that through the poles causes the 

 earth to be classed as a spheroid, 

 while minor irregularities have led 

 to the use of the term geoid, which 

 is equivalent to saying that the 

 earth is shaped like itself and like 

 nothing else. See Earth. 



Geok Tepe. Village in Turkis- 

 tan. It is 28 m. from Askhabad on 

 the Transcaspian Ely. It is noted 

 for its fortress, an immensely strong 

 building erected by the Turko- 

 mans, which in 1880 was stormed 

 by the Russians. 



Geological Society. British 

 learned society, the oldest geo- 

 logical society in the world. 

 Founded in 1807 and incorporated 

 by royal charter in 1825, it began 

 as a dining club, meeting at the 

 Freem \ eons' Tavern, Great Queen 

 Street, London, W.C. Rooms were 

 afterwi rds engaged at Garden 

 Court (Temple), Lincoln's Inn 

 Fields, Bedford Street (Covent 

 Garden), until, in 1828, apart- 

 ments were granted at Somerset 

 House, Strand. In 1874 the society 

 removed to its present quarters 

 at Burlington House, Piccadilly, 

 London, W. 



The society maintained a valu- 

 able museum of rocks, minerals, 

 and fossils until 1911, when the 

 collections were dispersed owing to 

 the growth of the library. British 

 collections were then presented to 

 the museum of practical geology 

 (Jermyn Street, London), and col- 

 lections from abroad to the British 

 Museum of Natural History (Crom- 

 well Road, London). The society 

 meets fortnightly from November 

 to June inclusive. Its publications 

 include the society's Quarterly 

 Journal, Abstracts of Proceedings, 

 and Record of Geological Litera- 

 ture. Two series of its valuable 

 quarto Transactions were pub- 

 lished between 1811 and 1856, but 

 none have been issued since. The 

 United States has a geological 

 society with aims similar to the 

 British one. Papers read before 

 it are published in the society's 

 Bulletin. 



Geological Survey. British 

 government department charged 

 with the investigation of the 

 geology of the British Isles and the 

 upkeep of the museum of practical 

 geology. The field work of the 

 survey officers includes a survey of 

 the rocks and soils, their mineral 

 contents, inquiries into questions 

 relating to water supply for the 

 dual purpose of the advancement 

 of science and the accumulation of 



3472 



facts useful to miners, farmers, 

 architects, builders, engineers, and 

 manufacturers. 



In 1815 William Smith issued 

 the first geological map of England, 

 and Sir Henry De la Beche followed 

 this great work by beginning to 

 plot the geology of the mining 

 areas of Cornwall and Devon on 

 the Ordnance Survey " one inch " 

 maps then being published. This 

 work, started as a private enter- 

 prise, soon received national re- 

 cognition, and in 1835 De la Beche 

 was "director of the ordnance 

 geological survey." From these 

 beginnings the department grew 

 under the successive control of 

 De la Beche, Sir R. J. Murchison, 



GEOLOGY 



and A. C. Ramsay, while other 

 distinguished geologists were also 

 connected with it. 



The department issues solid and 

 drift maps, memoirs on special 

 subjects, and maintains the geo- 

 logical collection which includes 

 remarkable relief models of Ingle- 

 borough and district, Assynt, etc. 

 The collection is housed at the 

 headquarters, the museum of 

 practical geology, Jermyn St., 

 London, S.W., a building erected 

 by the government and formally 

 opened by the Prince Consort in 

 1851. Students and inquirers are 

 welcomed at the museum which 

 is free to the public at certain 

 specified hours. 



GEOLOGY: THE READING OF THE ROCKS 



J. W. Gregory, F.R.S., Professor of Geology, Glasgow Univ. 



This work contains articles on all the important geological terms, 



e.g. Devonian; Pliocene ; Triassic, etc. Other cognate articles are 



Fossils ; Meteorites ; Rocks. See also Earth ; and biographies of 



Geikie and other geologists 



The word geology means literally 

 " a discourse on the earth " or " the 

 science of the earth." Geology is 

 the science which interprets the 

 evidence afforded by the materials 

 of the earth as to its composition, 

 structure, and history. It investi- 

 gates the minerals and rocks of the 

 earth's crust by aid of mineralogy 

 and petrology ; it determines the 

 mutual relations of the constituent 

 layers, which give the earth its 

 general structure by tectonic or 

 structural geology ; it studies the 

 forces which act on the earth by 

 dynamic or physical geology ; and 

 it compiles the history of the earth 

 from its beginning to the earliest 

 human records by historical or 

 stratigraphical geology. The last- 

 named is largely dependent on the 

 evidence of the remains of the suc- 

 cessive animals and plants which 

 have lived on the earth, and these 

 fossils are the subject matter of 

 palaeontology. 



ORIGIN AND GENERAL STRUCTURE 

 OF THE FJARTH. The origin of the 

 earth and of the other members of 

 the solar system was long attribu- 

 ted to consolidation from a hot 

 gaseous nebula the theory of 

 Laplace. In recent years the view 

 that has found most favour 

 among geologists is that the 

 earth was formed from vast swarms 

 of meteorites, which are scattered 

 through space in infinite numbers. 

 When a meteorite enters the earth's 

 atmosphere it becomes incandes- 

 cent, owing to the friction of the 

 air, and is then known as a shooting 

 star or meteor. Meteorites when 

 travelling through outer space are 

 extremely cold ; but in a dense 

 swarm they are heated by collisions 



and pressure and may be fused into 

 a compact body. 



The largest of the meteorites 

 known to us are composed mainly 

 of iron and nickel, with a small 



Eroportion of stony materials. 

 E a SAvarm of meteorites is 

 melted and formed into one mass, 

 the heavy metals would naturally 

 sink toward ths centre, and the 

 lighter stony matter would collect 

 on the surface and solidify as a 

 rocky crust ; the moisture would 

 condense on the surface in sheets of 

 water and the gases given off would 

 surround it as an atmosphere. This 

 arrangement occurs in the earth, 

 which consists of a central core sur- 

 rounded by three distinct layers. 

 The central core is known as the 

 centrosphere from its position, and 

 as the barysphere owing to the 

 heaviness of its constituents. It is 

 enclosed in the rocky crust of the 

 earth which is known as the litho- 

 sphere ; upon this in turn lies the 

 hydrosphere, which contains all the 

 waters near the earth's surface, 

 alike in the sea and rivers, in rocks 

 and in clouds. Surrounding the 

 whole is the gaseous envelope, the 

 atmosphere. 



The Centroaphere or Thermosphere 

 The centrosphere forms the 

 largest part of the earth, but as it is 

 inaccessible our knowledge of it is 

 gained indirectly, especially from 

 the study of earthquakes and mea- 

 surements of the weight of the 

 earth. Of this central mass two 

 facts are certainly known. First, it 

 is much hotter than the rocks on 

 the surface, since on descent into a 

 deep mine the temperature rises 

 and the water from deep springs is 

 hot. The increase of temperature 





