in Rock Magmas on passing from liquid to solid. 245 



zinc sulphate in water has been added for comparison. Here 

 the ordinates are ratios in terms of the resistance of the solu- 

 tion at 100°. Points above 100° were obtained under pressure. 

 Fig. 2 explains itself. 



A series of observations with another basic magma gave 

 corroborative evidence. The only electrical indication of 

 fusion was found in the case of the basalt (3). It was some- 

 what above 1100°, but is not certain. The rhyolitic or 

 acid magma (1) changes character suddenly below 500°, for 

 reasons probably accidental which must be further examined. 

 In this rock, which is very viscous even at 1600°, it is almost 

 impossible to free the magma of gas bubbles. We have not 

 been able to make allowance for volume expansion of the 

 magma, and the residual errors are in keeping with this omis- 

 sion. Finally, without sacrificing accuracy of temperature 

 measurement, it is difficult to make the electrodes sufficiently 

 large in area to exclude polarization. 



§ 3. The magmas. — The magmas investigated represent 

 three phases of igneous rocks from the same region. The 

 first is a rhyoiite; the second, a crystalline andesite or por- 

 phyrite ; and the third, a basalt. The chemical composition of 

 the rocks is shown by the following analyses made by Mr. J. E. 

 Whitfield. 



I. II. . in. 



Si0 2 . 75.50 61'50 48-49 



TiO„ . none none 2-19 



Al 2 O s 13-25 17-42 18.35 



Fe„0 3 1-02 4-66 7-63 



FeO -91 1-09 1-21 



MnO none tr. none 



CaO -90 5-33 10-40 



MgO -07 1-26 6-72 



Li 2 -06 -03 -02 



Na 2 4-76 3-99 3-02 



K 2 0__ 2-85 1-29 -57 



P 2 5 none -60 -20 



S0 3 7 . -32 -35 -52 



HO -41 2-44 -67 



100-05 99.96 99-99 



The three magmas clearly form the middle and extremes of 

 a chemical series. 



When heated in a platinum crucible, the basalt melts at 

 about 1250° to a fluid which at slightly higher temperatures 

 becomes quite liquid, and upon solidification forms a black 

 glass that is brown in the thinnest edges and is without gas 

 bubbles. 



