Researches on the Mineralogy of South America. 131 



not remain over one million and a half years above the sea- 

 level. 



It may therefore yet turn out that between 240,000 and 

 80,000 years ago might be the period of the glacial epoch, and 

 that those glacial epochs between 1,000,000 and 700,000 years 

 ago may belong to the Miocene period. 



With the view of ascertaining if the superior limit of excentri- 

 city was reached about 850,000 years ago, I determined the 

 values for one or two periods closely before and after this period, 

 but cannot find a higher value than that which has been assigned 

 to it. 



851,000 

 850,000 

 849,500 

 849,000 



0-07454 

 0-074664 

 00/466 

 007466 



There is another phenomenon necessarily connected with the 

 changes of the excentricity, viz. the submergence and emergence 

 of the land resulting from the displacement of the earth's centre 

 of gravity occasioned by the transference of the ice-cap from the 

 one hemisphere to the other, to which I shall at present simply 

 allude. About a year ago I stated, as an objection to this theory, 

 that the lowering of the ocean by the removal of the water to 

 form the cap would exceed the rise occasioned by the displace- 

 ment of the centre. Since then this objection has been re- 

 peatedly urged. There are, however, several considerations which 

 appear to have been overlooked by those who have discussed the 

 submergence question. When these are taken into account, it 

 will be found that the objection referred to is rather premature, 

 and that, even assuming the earth to be a rigid mass throughout, 

 a submergence to the extent of 100 or 200 feet is not only a 

 possible but a necessary effect. But the discussion of this point 

 must be deferred till another occasion. 



XVIII. Researches on the Mineralogy of South America. 

 By David Forbes, F.R.S., ipj* 



V. General Mineralogy of Chile. 



HAVING been enabled to devote much more time to the 

 exploration of Chile than to that of any of the other 

 countries of South America which I visited, I was occupied some 

 four years in traversing that country in all directions, from con- 



* Communicated by the Author. 



K2 



