136 ELEVATING CAUSES. 



character, while the union of several groups is called a 

 period. 



DELTA. This name, which was originally bestowed on the 

 deposits formed by the Nile at its mouth, which present the 

 triangular form of this letter of the Greek alphabet, is now 

 applied to all accumulations deposited at the mouths of 

 rivers, whether of this precise shape or not. 



DEGRADING AND ELEVATING CAUSES. There are two 

 opposite forces continually in operation on the crust of the 

 globe the one destructive, and the other conservative ; the 

 one consisting in the liability of rocks to be disintegrated 

 and carried away ; and the other, in the capability of the 

 materials which they have furnished to be again elaborated 

 into new strata. 



DEGEADING CAUSES. These are chiefly atmospheric, 

 fluviatile, or oceanic. The agency of the atmosphere is 

 twofold chemical and mechanical : the chemical action con- 

 sists in the absorption of oxygen and carbonic acid from the 

 atmosphere, and the dissolution of the rocks by such absorp- 

 tion. By these means they are speedily weathered, and, to 

 a certain extent, worn away. The mechanical effects consist 

 in the fissuring and cleaving of rocks by frost, and in its 

 abrading and carrying off the solid earth by the action of 

 rills, brooks, and streams, which, when swollen to floods, or 

 collected as rivers, exercise a powerful and destructive force. 



ELEVATING CAUSES. As the degrading powers are oc- 

 casioned by water, so the elevating causes are chiefly owing 

 to fire. They may be regarded as the rapid action of volca- 

 noes and earthquakes, and the slower agency of gradual 

 dynamic forces. The operation of volcanoes is too familiar 

 to require any lengthened explanation. Geologically speak- 

 ing, the action of a volcano is to raise earthy substances 

 from a low to a higher level. It often effects the most 

 sudden and important changes, and produces a hill on land, 

 or an island in the sea, in a single night. It frequently 

 upheaves the solid strata over a vast area; the coast of 

 Chili, along a line of a hundred miles, and extending 

 from the shore to the Andes, was raised many feet by the 

 earthquakes of 1822. Similar elevations are traced as having 

 occurred in various localities, during the early ages of the 

 earth. 



