28 DYNAMICAL GEOLOGY 



always acted with just the same intensity as they do to-day ; but 

 this assumption is neither necessary, nor in itself probable. 

 There is, on the contrary, much reason to believe that while cer- 

 tain forces act with greater efficiency at the present time than 

 they did in the past, others act with less. 



An attentive examination of the changes which are now in pro- 

 gress on the surface of the earth, will show us that nothing terres- 

 trial is quite stable or unchangeable, but that there is a slow, 

 ceaseless circulation of matter taking place on the surface and 

 within the crust of the globe. Matter, chemistry teaches us, is 

 indestructible, and, disregarding the relatively insignificant amount 

 of material which reaches us from outer space in the form of 

 meteorites, the sum total of matter composing the globe remains 

 constant. But while practically nothing is added to or taken away 

 from the materials which make up the earth's crust, ceaseless 

 cycles of change continually alter the position, physical relations, 

 and chemical composition of those materials. This circulation of 

 matter may be aptly compared to the changes which take place 

 in the body of a living animal, only, of course, they are of a differ- 

 ent kind and are effected at an infinitely slower rate. In the ani- 

 mal body, so long as life lasts, old tissues break down into simpler 

 compounds and are gotten rid of, while new tissues are built up 

 out of fresh material. So, on the earth rock masses decay, their 

 particles are swept away, accumulate in a new place, perhaps far 

 distant from their source, and are consolidated into new rocks, 

 which in their turn are attacked and yield materials for further 

 combinations. The study of the physical and chemical changes 

 in the bodies of animals and plants constitutes the science of 

 physiology, and by analogy we may call dynamical geology the 

 physiology of the earth's crust. Analogies, however, must not 

 be pushed too far, or they land us in absurdities. One essential 

 difference between the earth and a living organism suggests itself 

 at once, namely, that the former is self-contained, and neither 

 ejects old material nor receives new, but employs the same matter 

 over and over again in ever-varying combinations. The animal or 

 plant, on the contrary, continually takes in new material from 



