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., Sig, 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 clip 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., Elq, dis double, and icrepov, 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, 
