ADVISORY COMMITTEE ON GEOPHYSICS 6 1 



laws of magnetic variation were more accurately known, we should 

 possess a curious new chronological method. French naturalists 

 have begun to examine baked clays from the lava fields of the 

 Auvergne, and in this case declination, as well as magnetic meri- 

 dian, can be examined. 



These French observations, contained in the Comptes Rendus for 

 1 89 1 are a small beginning of a great task. This task is rendered 

 somewhat more difficult by the circumstance that in taking samples 

 of baked clays their present geographical position, say beneath a 

 coulee of lava, cannot be accurately fixed by compass, the needle 

 being disturbed by surrounding masses, so that true north in every 

 case must be fixed by simple astronomical methods. It is quite 

 superfluous to say that intense interest would be awakened by the 

 examination of such baked clays from high latitudes, say, from the 

 basaltic region of Disko, or any boreal district. All questions 

 relating to a variability in the position of the poles, etc., are thus 

 raised. So, too, a series of observations from superposed beds of 

 one and the same region, for example the Cascades, would be highly 

 instructive, and a series of observation? from the West Indies or 

 Mexico to boreal districts would be highly meritorious work, and, 

 so far as I can see, within the scope of your institution. Perhaps 

 this may in future prove to be the way to attain a positive chro- 

 nology^ for certain geological events, but the seemingly periodic 

 character of magnetic variation will, I fear, turn out a difiiculty 

 which only the coming generation may overcome. 



I remain, dear sir, respectfully, and full of sincere envy, 

 Yours, 



E. SuESS. 



[Z)r. Ernst Mach to E. Siiess, May 2p, ipo2.'] 



Vienna, May 2p, igo2. 

 My Dear Mr. President : 



I transmit herewith the letter which I was unable to give you 

 yesterday, and also a few lines on the question there touched upon: 



Hoiv can Geophysics best be promoted f The question is easier 

 put than answered. It cannot be denied, however, that the deter- 

 mination of the effects which the highest attainable pressures, as 

 well as the highest and lowest attainable temperatures, exert upon 

 the melting point or the freezing point, the latent heat, the specific 



