1876.] Geological Changes on the Earth's Axis. 331 



are all on the same complete meridian and all in latitude 45°. A Table 

 of numerical results depending on the values o£ certain elliptic functions 

 is given. 



4. In this part an endeavour is made to collect evidence as to the 

 extent to which the earth may have undergone deformation from geolo- 

 gical changes. The object is to discover what are the largest areas over 

 which there has been a consentaneous rise or fall, and what is the great- 

 est vertical amount of that rise or fall ; also to determine how the 

 erosion of the land and the sea affect the local excesses or deficiencies of 

 matter on the earth's surface. The areas and amounts of elevation and 

 subsidence which on a sealess and rainless globe are equivalent, as far 

 as producing excesses or deficiencies of surface matter, to those which 

 obtain on the earth are referred to as "effective;" and it is only the 

 effective elevation or subsidence which we require to know in order to 

 determine the shift of the earth's axis. 



The evidence as to area is very meagre, because precise boundaries to 

 regions of elevation and subsidence cannot be assigned ; but, faute cle 

 mieux^ the author's father, Mr. Charles Darwin, marked out for him on 

 a map an area in the Pacific Ocean which (on account of the structure 

 of the coral islands) he believes to have undergone subsidence within a 

 recent geological period. From a consideration of this and of other 

 points the author believes that from to -Jq of the whole earth's 

 surface may, from time to time, have undergone elevation and sub- 

 sidence. The greatest vertical effective amount of rise or fall cannot be 

 determined from geological evidence, because of the effects of erosion 

 and of the influx of the sea into parts below the mean level of the earth. 



The only way of determining the point seems to be to find what is the 

 difference of mass, standing on unit area of the earth's surface, in an 

 ocean of, say, 15,000 feet deep, and in land of, say, 1].00 feet high. 

 Erom this difference of mass the effective elevation of an ocean-bed in 

 its conversion into land can be at once determined. Taking the above 

 numbers, it is found to be 10,436 feet ; and in the examples given in the 

 following part, the deflection of the polar axis, for an assumed effective 

 elevation of 10,000 feet, is given in each case. 



It is then pointed out that if the deformation of the earth were of 

 very wide extent, the level surface of the sea would approximately follow 

 the rocky surface, and that thus there might be sufficient change in the 

 earth's shape to sensibly affect the position of the principal axis, without 

 there being any geological signs of elevation or subsidence. 



5. Numerical application is now made of the preceding work to the 

 case of the earth, and, as before stated, all the results are given for 10,000 

 feet of effective elevation. 



The first application is to continents and seas of maximum effect, and 

 a Table of results is given. It may be here stated that if ^-J-o of the 

 earth's surface is elevated, the deflection of the pole is 11^'; if -^^^ 



