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VIII. On the Influence of Geological Changes on the Earth's Axis of Botation. By 
George H. Darwin, M.A., Fellow of Trinity College , Cambridge. Communicated 
by Professor J. C. Adams. 
Eeceived October 13, — Read November 23, 1876. 
The subject of the fixity or mobility of the earth’s axis of rotation in that body, and 
the possibility of variations in the obliquity of the ecliptic, have from time to time 
attracted the notice of mathematicians and geologists. The latter look anxiously for 
some grand cause capable of producing such an enormous effect as the glacial period. 
Impressed by the magnitude of the phenomenon, several geologists have postulated a 
change of many degrees in the obliquity of the ecliptic and a wide variability in the 
position of the poles on the earth ; and this, again, they have sought to refer back to 
the upheaval and subsidence of continents. 
Mr. John Evans, E.R.S., the late President of the Geological Society, in an address 
delivered to that Society, has recurred to this subject at considerable length. After 
describing a system of geological upheaval and subsidence, evidently designed to produce 
a maximum effect in shifting the polar axis, he asks : — “ W ould not such a modification 
of form bring the axis of figure about 15° or 20° south of the present, and on the 
meridian of Greenwich — that is to say, midway between Greenland and Spitzbergen % 
and would not, eventually, the axis of rotation correspond in position with the axis of 
figure 1 
“ If the answer to these questions is in the affirmative, then I think it must be conceded 
that even minor elevations within the tropics would produce effects corresponding to 
their magnitude, and also that it is unsafe to assume that the geographical position of 
the poles has been persistent throughout all geological time.”* 
On the few occasions on which this subject has been referred to by mathematicians, 
the adequacy of geological changes to produce effects of such amount has been denied. 
Amongst others, the Astronomer Royal and Sir William Thomson have written briefly 
on the subjectf, but, as far as I know, the subject has not hitherto been treated at much 
length. 
The following paper is an attempt to answer the questions raised by Mr. Evans ; but 
as I have devoted a section to the determination of the form of continent and sea which 
would produce a maximum effect in shifting the polar axis, I have not taken into consi- 
deration the configuration proposed by him. 
The general plan of this paper is to discuss the following problems : — 
* Quart. Journ. Geol. Soc. 1876, xxxii. Proc. p. 108. 
2 Q 
MDCCCLXXVII. 
f In papers referred to below. 
