EUTECTICS IN ROCK MAGMAS 93 



suggested, and in part only confirmatory of the suggestions 

 above. For instance, Linck (pp. 606, 607) found a magma 

 with 49.05 SiOg and an alkali silica ratio of O.i 15, after absorbing 

 all the silica it could at 1300° C. change to a magma with 52.20 

 SiOg and an alkali silica ratio of 0.082, almost precisely on the 

 supposed eutectic line. Another with 62.62 per cent, silica 

 absorbed a mixture of ferrous and ferric iron until the silica per- 

 centage dropped to 60.58 and the magma could be nearly 

 expressed as (NaKj^ O • AU O 3 ■ 6 SiO^ + >^ CaO • Al^Og ■ 

 2 Si03+3 (Ca Mg, FeO) SiOg. 



In Schweig's extensive series of experiments, starting with a 

 strongly alkaline magma (17.5 per cent, alkali molecules), then 

 adding separately silica, alumina, iron, magnesia, and lime to 

 saturation, and then later adding also silica to saturation with the 

 other oxide, only in case silica is added to saturation do we find 

 analyses which are comparable with any of the rock analyses 

 plotted by Iddings. The inference is suggested that the natural 

 igneous magmas of the alkaline group are always able to absorb 

 all the silica they will take. 



^ When magnesia or iron oxide or alumina is added with the 

 silica the alkali-silica ratio drops only to about 0.15. But the 

 magma is much more capable of absorbing lime, and after satura- 

 tion with lime and silica the alkali-silica ratio becomes 0.895 ^"^^ 

 the SiOg 70 per cent., bring it well into the eutectic belt, which 

 apparently therefore is that of magmas saturated with lime and 

 silica. 



All the glasses were cooled as quickly as possible, and no 

 attempt was made to determine the temperatures of solidification. 

 It would be interesting to repeat the experiments and note the 

 latter. 



Alfred C. Lane. 



Lansing, Mich. 



