SUMMARY OF DYNAMICAL GEOLOGY 185 



in the rocks. The materials of the earth's crust we have found 

 to be in a state of ceaseless, though very slow, circulation, disin- 

 tegrating here, accumulating there. Volcanoes bring up from the 

 interior of the earth molten and fragmental materials, which con- 

 solidate into glassy or crystalline rocks, or beds of cinders and 

 tuffs. Earthquakes shatter the rocks and shake down masses from 

 the cliffs, while changes of level bring the sea over the land, or 

 raise parts of the sea-bottom into land surfaces. 



Everywhere over the land and along the seacoast processes of 

 disintegration are at work upon the rocks, decomposing them 

 chemically and breaking them up mechanically. Rain, wind, frost, 

 changes- of temperature, underground waters, rivers, glaciers, the 

 currents and waves of the sea, all take part in this work, each in 

 its own characteristic way. The products of this destruction are 

 transported by various agents, especially the rivers, to lakes and 

 the sea, though some find a resting-place, for a longer or shorter 

 time, upon the land. Part of this material is dissolved in water, 

 but the greater part is mechanically suspended. The suspended 

 portion is sorted by the power of water and laid down in sheets 

 and layers upon the beds of lakes or the ocean, forming stratified 

 masses, while dissolved materials are largely extracted by the 

 agency of animals and plants, and deposited under water as accu- 

 mulations of calcareous or siliceous sediments. By various pro- 

 cesses, these incoherent and loose masses are consolidated ; they 

 may be upheaved to form new land surfaces, and a new cycle of 

 destruction and reconstruction will begin. These changes we 

 have studied in order to obtain a key for the interpretation of 

 the earth's history, which is recorded in the rocks, and we have 

 found that these records may be so interpreted by the aid of pro- 

 cesses which are still at work. We have yet much to learn, how- 

 ever, before such a systematic history can be attempted, and first 

 to study the ways in which the rocks are actually arranged and 

 the disturbances which they have undergone : this is structural 

 geology, the next division of our subject. 



