lo Str0MEV1';k, The Formation of Minerals in Gi'anite. 



dp the increased pressure is expressed in i^rams per 

 sq. cm. One atmosphere is i,033'4 grams per sq. cm. ; in 

 the table the value i,000 grams has been adopted. As 

 X is in the denominator, and as it is believed tobever\dow 



for most minerals of the earth's crust, the value of — will 



dp 



be large. Its sign is determined by the change of volume 



when melting. Its value will also vary with the 



pressure, unless the elasticity of the solid and the liquid 



are equal ; it may even be reversed under high pressures. 



The determination of these various values for quartz 

 and other minerals is attended with great difficulty. 

 Professor Joly, as already mentioned, measured the 

 changing volume of basalt, augite and orthoclase. The 

 apj^aratus consisted of a sheet of platinum, bent into a 

 cylindrical shape and heated to definite temperatures by 

 an electric current; a small bead of mineral about ^V inch 

 in diameter was suspended in the hollow, and its changing 

 diameter with rising temperature was measured with a 

 long-focus microscope. His results and those obtained 

 by Mr. Barus on cooling diabase are plotted as curves in 

 Fig. 2. 



The upward bends in the temperature-volume 

 curves is ver\- marked ; apparently they do not coincide 

 with the melting points, which are stated to be 1,090 for 

 the basalt specimen, and 1,190 (stick)') for the diabase. 

 If only similar experiments could be made on quartz, the 

 question whether the formation of granite can be explained 

 by the influence of pressure on the melting point would 

 speedily be settled. Unfortunately, cjuartz does not melt 

 until i,400°C has been reached, at which temperature the 

 platinum electric furnace would collapse ; besides, silica 

 seems to act on platinum at high temperatures. Possibly 

 Mr. Rarus' method might lead to results. Should quartz 



