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Denudation. The carrying away by the action of running water 

 of a portion of the solid materials of the land, by which in- 

 ferior rocks are laid bare. Etym., dc?wdo, to lay bare. 

 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 projecting, like a wall, from the softer strata on 

 both sides of them having wasted 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 generally of larger dimensions, and 

 have their sides parallel for considerable distances ; while veins 

 have generally many ramifications, 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 or dip of inclination. 



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, 

 England, Spain, and other countries. 



Eocene. The great tertiary era is divided into four periods, the first 

 of which is called Eocene, Ew?, aurora, and Kecivo?, recent, indi- 

 cating that in the beds of this division, we see the first traces 

 or dawn of the present order of things. The class of fossils 

 most serviceable in determining the relations of the existing to 

 the extinct species, are shells, and it is between these, more 

 particularly that the comparison has been made. Out of about 

 1200 shells discovered in Europe in this lower division of the 

 tertiary rocks, 38 only are identical with species known to be 



