GLOSSARY. 



67 



DELTA. When a great river before it enters the sea divides into 

 separate streams, they often diverge and form two sides of a 

 triangle, the sea being the base. The land included by the three 

 lines, and which is invariably alluvial, is called a delta from its 

 resemblance to the letter of the Greek alphabet which goes by 

 that name A. Geologists extend the boundaries of the delta, 

 so as to include all the alluvial land outside the triangle, which 

 has been formed by the river. 



DENUDATION. The carrying away of a portion of the solid materials 

 of the land, by which the inferior parts are laid bare. Etym,, 

 denudo, to lay bare. 



DESICCATION. The act of drying up. Etym., desicco, to dry up. 



DIAGONAL STRATIFICATION. For an explanation of this term, see 

 vol. iii. p. 174. 



DICOTYLEDONOUS. A grand division of the vegetable kingdom, 

 founded on the plant having two cotyledons or seed-lobes. 

 Etym., &e, dis, double, and cotyledon. 



DIKES. "When a mass of the unstratified or igneous rocks, such as 

 granite, trap, and lava appears as if injected into a great rent in the 

 stratified rocks, cutting across the strata, it forms a dike ; and 

 as they are sometimes seen running along the ground, and pro- 

 jecting, like a wall, from the strata on both sides of them being 

 worn away, they are called in the north of England and in 

 Scotland dikes, the provincial name for wall. It is not easy to 

 draw the line between dikes and veins. The former are gene- 

 rally of larger dimensions, and have their sides parallel for con- 

 siderable distances ; while veins have generally many rami- 

 fications, and these often thin away into slender threads. 



DILUVIUM. Those accumulations of gravel and loose materials 

 which, by some geologists, are said to have been produced by 

 the action of a diluvian wave or deluge sweeping over the sur- 

 face of the earth. Etym., diluvium, deluge. 



DIP. When a stratum does not lie horizontally, but is inclined, the 

 point of the compass towards which it sinks is called the dip of 

 the stratum, and the angle it makes with the horizon is called 

 the angle of dip or inclination. 



DIPTERA. An order of insects, comprising those which have only 

 two wings. Etym., &e, dis double, and Trrepov, pteron, wing. 



DOLERITE. One of the varieties of the trap-rocks, composed of 

 augite and felspar. 



DOLOMITE. A crystalline limestone, containing magnesia as a con- 

 stituent part. Named after the French geologist Dolomieu. 



DUNES. Low hills of blown sand that skirt the shores of Holland, 



