The Evolution of the Sciences 



These determinations seem to indicate that 

 the thermic progression is very regular, and do 

 not show any tendency for it to decrease at the 

 extreme depths. It therefore seems incontest- 

 able that the temperature must continue in- 

 creasing to much greater depths. The tempera- 

 ture at a depth of about four miles will certainly 

 be, as stated by M. de Lapparent, very near 170 

 centigrade. But does it ever rise high enough 

 to melt the deep-seated rocks? We must 

 examine this question in detail. 



A rise of i every 38 \ yards would lead 

 to a temperature of 2000 centigrade at a depth 

 of about forty-four miles, and such a tempera- 

 ture is sufficient to melt in the retorts of our 

 laboratories all the metals and nearly all re- 

 fractory materials. The melting temperatures 

 of the principal rocks are known to-day with 

 accuracy, thanks especially to the researches 

 carried out by the U.S.A. Geological Survey. 

 Basalt, the commonest constituent of the lavas 

 erupted by volcanoes, melts at 880, and the 

 components of granite, mica, felspar and quartz 

 have their melting points at 1440, 1520 and 



1775 C. 



130 



