DYNAMICAL GEOLOGY 29 



without, in the shape of food, and ejects the waste resulting from 

 the breaking down of tissue. 



Although the earth needs no fresh supplies of matter, its dynami- 

 cal operations are, to a very large extent, maintained by energy from 

 without ; namely, from the sun. The circulation of the winds and 

 waters, the changes of temperature, and the activities of living 

 beings, all depend upon the sun's energy, and were that with- 

 drawn, only such changes as are brought about by the earth's 

 internal heat could continue in operation. 



The study of dynamical agencies, subterranean and surface, 

 necessarily gathers together an enormous mass of detail. But we 

 need concern ourselves with only so much of this as throws light 

 upon the earth's history, so that the sciences of dynamical geology 

 and physical geography, though having much in common, are 

 essentially distinct. In order to make clear the operations of the 

 forces which tend to modify the surface of the earth, it is neces- 

 sary that we should classify and arrange them, so that they may 

 be treated in a more or less logical order. However, in making 

 such a classification, it is impossible to avoid entirely a certain 

 arbitrariness of arrangement, since we must consider separately 

 agencies that act together. Natural phenomena are not due 

 to single causes, but to combinations and series of causes, and yet, 

 to make them intelligible, they must be treated singly or in simple 

 groups, else we shall be confronted by a chaotic mass of uncorre- 

 lated details. The career of a raindrop, from its first condensa- 

 tion to its entrance into the sea through the mouth of some river, 

 is a continuous one, yet rain and rivers are distinct geological 

 agencies and do different kinds of work. Again, the very im- 

 portant way in which the various dynamical agents modify, check, 

 or augment one another, must not be overlooked in a systematic 

 arrangement of these agents. 



Some of the agencies that we shall consider may seem, at first 

 sight, to be very trivial in their effects, but it must be remembered 

 that they appear so only because of the short time during which 

 we observe them. For enormously long periods of time they 

 have been steadily at work, and their cumulative effects must 



