MiDicJicstcr Memoirs, Vol. xliv. (1900), No. I. 



7 



per 450 feet of depth, with the other substance it is about 

 I C. per 300 feet of depth. Assuming now, for instance, 

 that these rates also applied to felspar and quartz respect- 

 ively, and starting with the melting temperatures as given 

 in Tabic /., remembering also that the mean temperature 

 gradient in the earth's crust is about i*-^C. per 1 10 feet, we 

 can construct Table III. which gives hypothetical melting 

 points for quartz and felspar at varying depths down to 

 200,000 feet. 



Tai;le III. 



Although these values are purely imaginary, except 

 for the depth o, they are not at all impossible ones; 

 Maxwell* even assumes rates of increase of melting 

 temperatures exceeding the earth's temperature gra- 

 dient. Besides, both napthylamine and paratoluidine 

 raise their melting point under pressure, and it is only 

 the difference in their two rises which has in this example 

 been taken into account ; if, however, the melting point 

 of quartz were to sink under pressure, while that of 



* Theory of Heat, iS88, p. 21. 



