292 ANNUAL OF SCIENTIFIC DISCOVERY. 



supposed, viz., that the earth is not a rigid body, the first day's whirl would 

 make the new position of the axis again a permanent position, permanent, 

 at least, until again disturbed by the upheaval of another mountain mass, 

 capable of producing another change. I hope I have correctly represented 

 what Mr. Airy admits, and that we have now got the problem into a form 

 which will enable us very readily to discuss it. 



And first, as to the magnitude of mountain masses as they now exist, I 

 grant that there is no existing mass that can produce more than a scarcely 

 appreciable change. But the present mountain masses represent but frag- 

 ments of their former bulks. The great range of the Andes, with the whole 

 continent of South America, is probably the most recently elevated moun- 

 tain range upon the face of the earth; the whole has been raised to its 

 present great height of about five miles within the most recent geological 

 period, and the greater part of it has been raised several feet within our own 

 time. And I know of no reason why it should not continue to rise until it 

 reached a magnitude which would produce a sensible change in the position 

 of the axis of the earth. But although it simplifies the conception of the 

 problem to suppose the elevation of one mountain mass, we need not depend 

 upon the elevation of one only for producing the effect; for if, with Sir C. 

 Lyell, we suppose other configurations of the continents and seas, and that 

 another great continent, with its mountain ranges, like the Himalayas, rose 

 simultaneously with South America at the other side of the globe, whilst 

 other lands in opposite directions are sinking, it is obvious that under this 

 supposition the effect might be quadrupled. 



But, it may be asked, can this elevation of mountain masses be supposed 

 to continue to proceed to an indefinite extent? Certainly not; their attaining 

 a certain magnitude would lead to results which would rapidly produce the 

 reduction of their bulk, if it did not sweep them away altogether. 



The great geological epochs of which we have been treating are separated 

 by intervals of what may be called the periods of tranquil deposition of 

 nearly horizontal strata, antl those periods of disturbance during which the 

 strata have been thrown into great systems of undulations, and whole 

 races of animals and plants utterly and simultaneously annihilated over 

 the whole world, and the climate changed for the succeeding period of tran- 

 quillity. 



These tranquil periods have been of such long duration, that no geologist 

 ventures to say what number of years they occupied: a period of millions 

 of years ago is but as yesterday in geological chronology, and the vast 

 periods of time required for mountains to attain a magnitude sufficient to 

 disturb this state of tranquillity is necessary to, and consistent with, the 

 hypothesis. 



We must not investigate this problem, therefore, under the supposition 

 that vast mountain masses have suddenly made their appearance in any 

 given positions. Their increase, as measured by the chronology of man, is 

 so slow that we could not possibly expect that astronomical observation 

 could detect the effect of the increase in their magnitude in disturbing the 

 position of the axis of the earth. The effect of this slow increase may, by a 

 rough analogy, be compared with that which takes place when the base of a 

 great iceberg is slowly dissolved, until eventually the whole mass gives a 

 roll, and settles into a new position of equilibrium. 



Let us suppose that a mountain mass may possibly have attained lo the 

 required magnitude to produce the change, and that at the critical period or 



