764 



DENTITION 



DENUDATION 



Great Britain and Ireland is L.D.S. (Licentiate in 

 Dental Surgery ). The only foreign degrees ( both 

 American) qualifying for registration in Britain 

 are D.M.D. (Harvard) and D.D.S. (Michigan). 

 In 1888 there were in practice in Great Britain and 

 Ireland 977 licentiates under the new act, an,d 3889 

 practitioners registered as having been in practice 

 before 1878. 

 Dentition. See TEETH, TEETHING, HORSE. 



D'Entrecasteaux Islands, since 1884 part 

 of the British protectorate of New Guinea, lie north 

 of the south-eastern extremity of New Guinea. 

 With an area of 1083 sq. in., they comprise three 

 chief islands separated by narrow channels. They 

 are named after the French admiral and explorer, 

 Bruni D'Entrecasteaux (1739-93), who visited these 

 waters in 1792. His name is also preserved in 

 D'ENTRECASTEAUX POINT on the south-west coast 

 of Western Australia ; and in D'ENTRECASTEAUX 

 CHANNEL, separating the south of Tasmania from 

 Brune Island. 



Denudation, in Geology, means the laying 

 bare of underlying rocks by the removal of super- 

 ficial matter, and also the process by which the 

 earth's surface is broken up and the loose material 

 carried away. The more important agents of 

 denudation are wind, rain, .running water (springs, 

 underground streams, brooks, and rivers), frost, 

 snow, ice (glaciers), the sea (waves, breakers, 

 currents), plants, and animals. The action of 

 wind is seen in the erosion of rocks produced by the 

 sand and grit which in certain dry regions is swept 

 by the winds against projecting rocks, an action 

 Often resulting in the undermining of cliffs and the 

 downfall of rock-masses. The sand produced by 

 the superficial disintegration of rocks is carried 

 forward and heaped up in the form of dunes or 

 sandhills (see DRIFT). Rain is also a potent agent 

 of denudation, its action being both chemical and 

 mechanical. Rocks are more or less altered and 

 decayed, and the decomposed materials carried off 

 in solution by rain-water. The more soluble rocks, 

 such as rock-salt, gypsum, and limestone, readily 

 succumb, but there are very few rocks indeed 

 which are not more or less acted upon chemically 

 by rain. So that in many places the rocks are 

 thus ' weathered ' to considerable depths, the 

 decomposed crusts varying in thickness from a 

 mere line up to many feet or even yards. The 

 mechanical action of rain consists chiefly in the 

 sweeping away of this disintegrated material, 

 which often accumulates in hollows, forming what 

 is called rain-wash. Running water acts also 

 chemically and mechanically. Thus, the rain that 

 sinks underground and rises to the surface again in 

 springs, brings about many changes in rocks. 

 Immense quantities of mineral matter are brought 

 up in solution, and thus, in time, underground 

 cavities are formed, especially in the more soluble 

 rocks. In regions of calcareous rocks, the whole 

 drainage is sometimes conducted underground, the 

 ingulfed streams and rivers acting both chemi- 

 cally and mechanically, and giving rise to a large 

 series of subterranean tunnels (see CAVE.) The 

 action of underground water often brings about 

 local subsidences, falls of rock, and great landslips. 

 The denuding action of superficial terrestrial waters 

 is seen in the excavation of gullies, ravines, and 

 river-valleys, one of the most remarkable examples 

 of river erosion being the Grand Canon of the 

 Colorado (q.v. ), which is a chasm nearly 400 miles 

 long, with approximately vertical walls rising to a 

 height of 4000 to 7000 feet above the bed of the 

 river. 



Frost acts with great intensity at high levels and 

 in high latitudes, but even in temperate regions its 

 action is very marked and productive of great disin- 



tegration of rocks. Indeed, in the production of 

 the weathered crusts of rocks, frost is hardly less 

 active than rain. It is in arctic and mountain- 

 ous countries, however, that its action is most 

 conspicuous. The rocks under its influence are 

 ruptured and shattered to such a degree that 

 frequently the parent rock-masses become buried 

 under shivered heaps of their own debris. Glaciers 

 are likewise powerful denuding agents. They are 

 not only instrumental in transporting the rock- 

 rubbish which is showered down upon them from 

 overhanging cliffs, &c. (see BOULDERS), but by 

 means of the blocks and debris which they 

 drag forward on their beds, they grind, furrow, 

 and smooth the rocks over which they flo\v. 

 The peculiarly muddy character of the water 

 that escapes from the terminal front of a 

 glacier shows how powerful this erosive action 

 must be, for the mud carried in suspension is 

 simply the fine flour of the rocks which has resulted 

 from the grinding action. The sea, again, acts 

 like a great horizontal saAV, which is continually 

 rasping away the rocks along the coast. Cliffs are 

 in this way undermined, rock-falls take place, and 

 the tumbled rock-masses are by-and-by pounded 

 down into shingle, gravel, and sand, which are 

 hurled by the waves against the cliffs, and thus ere 

 long the latter is again undermined, and further 

 rock-falls take place. 



The chief denuding agents have now been men- 

 tioned, but the destructive action of plants and 

 animals cannot be ignored. Thus plants aid in the 

 demolition of rocks by sending their roots into rock- 

 crevices and wedging the masses asunder, and hence 

 they aid the freer percolation of water, and prepare 

 the way for the better action of frost. Vegetation 

 also, by attracting rain, tends to increase the flow 

 and erosive action of streams and rivers, while 

 its decomposition yields to rain those organic 

 acids which so greatly increase the chemical 

 action of that agent. The destructive action of 

 animals, again, is seen in the weakening of rocks 

 on a sea-coast produced by the drilling and boring 

 of Saxicava, Pnolas, &c., and by annelids, echini, 

 and sponges. Rocks so weakened fall more readily 

 before the battering of waves and breakers. Thus, 

 the whole surface of the land, from the summits of 

 mountains down to the sea-coast, is subject to 

 denudation. Little change may be perceptible in a 

 lifetime or even during many centuries, but an ex- 

 amination of the rocks shows that many thousands 

 of feet of solid strata have been gradually removed 

 from the surface of a country. Thus, in many 

 districts where faults occur, no ineqxiality at the 

 surface betrays the presence of dislocations ; the 

 whole area has been reduced by denudation to the 

 same level, hundreds or even thousands of feet of 

 strata having been removed from the upcast side 

 of the faults (see DISLOCATION). Some attempts 

 have been made to form a rough estimate of the 

 rate at which the general surface of the land is 

 reduced by denudation. As the sediment of gravel, 

 sand, and mud which a river carries down to the 

 sea represents the actual loss sustained by the sur- 

 face of the area drained by it and its tributaries, it 

 is obvious that if we could correctly estimate the 

 amount of sediment transported to the sea by the 

 rivers of any given area, we should at the same 

 time ascertain the rate at which that area ia 

 denuded. Observation has shown that this denuda- 

 tion proceeds more rapidly in some regions than in 

 others, and, therefore, that the work of no in- 

 dividual river can be taken as a standard by which 

 to estimate the general rate of erosion all the world 

 over. Much depends on physical and climatic con- 

 ditions, and much on the geological structure of a 

 country and the composition of its prevalent rock- 

 masses. Thus, the Mississippi is said to remove 



