INTERIOR FORCES OF THE EARTH. 115 



the first step in this direction is a clear statement of the problems to be 

 solved. The object of this address is to contribute something, however 

 small, to such clear statement. 



EFFECTS OF INTERIOR FORCES. 



As the interior of the earth is inaccessible to direct observation, we 

 can reason concerning interior forces only by observation of their effects 

 on the surface. Now these effects, as usually treated, are of three main 

 kinds : (1) Volcanoes, including all eruptions of material from the in- 

 terior; (2) Earthquakes, including all sensible movements, great- and 

 small; (3) Gradual, slow movements affecting large areas, imperceptible 

 to the senses, but accumulating through indefinite time. 



It is certain that of these three the last is by far the most fundamental 

 and important, being, indeed, the cause of the other two. Volcanoes 

 and earthquakes, although so striking and conspicuous, are probably 

 but occasional accidents in the slow march of these grander movements. 

 It is only of these last, therefore, that we shall now speak. 



KINDS AND GRADES OF EARTH-CRUzT MOVEMENTS. 



The movements of the earth's crust determined by interior forces are 

 of four orders of greatness : (1) Those greatest, most extensive, and prob- 

 ably primitive movements by which oceanic basins and continental 

 masses were first differentiated and afterward developed to their present 

 condition; (2) Those movements by lateral thrust by which mountain 

 ranges were formed and continued to grow until balanced by exterior 

 erosive forces ; (3) Certain movements, often over large areas, but not 

 continuous in one direction, and therefore not indefinitely cumulative 

 like the two preceding, but oscillatory, first in one direction, then in an- 

 other, now upward and then downward ; (4) Movements by gravitative 

 readjustment, determined by transfer of load from one place to another. 

 Perhaps this last does not belong strictly to pure interior or earth-derived 

 forces, since the transfer of load is probably always by exterior or sun- 

 derived forces. Nevertheless the} 7- are so important as modifying the 

 effects of other movements and have so important a bearing on the in- 

 terior condition of the earth that they cannot be omitted in this con- 

 nection. 



Now of these four kinds and grades of movement the first two are 

 primary and continuous in the same direction, and therefore cumulative, 

 until balanced by leveling agencies. The other two. on the contrary, 

 are not necessarily continuous in the same direction, but oscillatory. 

 They are, moreover, secondary, and are imposed on the other two or 



