ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 307 



Silica .,48.22 



Alumina 16.35 



Protoxide of iron . 10. 75 



Lime 19 . 29 



Magnesia .„ . , 3.23 i 35.28 



Protoxide of copper 75 



" « zinc 1.2.6 



99.85. 



According to Plattner, the melting point of these slags is about 

 1400 ° Centigrade.!} - If we suppose that the increase of tem- 

 perature downward in the earth's crust progresses at the rate of 

 1 ° C. for every 100 feet, the thickness of the earth's crust may 

 be calculated as follows. The temperature of the Mondorff arte- 

 sian well was 34 ° C, at a depth of 2066 feet. If we add 100 

 %>t for each of the remaining 1366 ° ( 136,600 ft.)— the tem- 

 perature of 1400° would exist at a depth of 138,666 feet, (26£ 

 English, or 22§ geographical miles.) However crude and un- 

 certain this method of calculating the thickness of the earth's 

 crust may be, it appears nevertheless to have been almost the 

 only one hitherto employed for that purpose. It seems to have 

 been assumed on all hands that the increase of temperature takes 

 place in the ratio of a simple arithmetical progression. Hum- 

 boldtj adopts the idea that " granite is in a state of fusion about 

 " 26 or 30 geographical miles beneath the surface. 1 ' At another 

 place§ he states it at " somewhat more than 20 geographical 

 " miles (2l r \ = 25 English)." "45,000 metres= 24 geographical 

 " miles, was named by Elie de Beaumont (Geologie, edited by 

 '* Vogt, 1846, I, 32) as the thickness of the solid crust of the 

 a earth. Bischof (Warrnelehre des Innnern unseres Erdkor- 

 " pers, pp, 271 and 286,) estimated it between 122,590 feet and 

 " 136,448 feet, or on the average 21J- geographical=24j English 

 " miles." The average diameter of the earth being 6864 miles, 

 it follows from the above estimate, that the thickness of the 

 earth's crust only amounts to about 777th of the radius of its 

 circumference. When we reflect on this result, it would appear 

 that this thickness is altogether insufficient to lend to the earth's 

 crust that stability which it now possesses. Moreover, there are 

 other estimates than those above quoted, which give to the earth's 



* Kerl. Handbuch der Huttenknude, I, 296. 



t Ibid ; I, p. 282. J Kosmos ; English edition, I, 26. § Ibid V, 169? 



