MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 241 



to the base of 49, 50, 51, and in one single crystal up to 56 mm., 

 and of a thickness parallel to c of 13-14 mm. (Fig. 54). The slags 

 contained about 90 per cent melilite. It may be noted that a 

 meliHte melt with some per cent of FeO is very thin. 



In another slag, also containing melilite, rich in FeO (from 

 Przibram, 1913), the lateral edge of the quadratic crystals measured 

 22 mm. 



The largest crystals of akermanite, found by me in a slag con- 

 taining about 90 per cent crystallized mineral (with about 4 per 

 cent AI2O3), occurred in 

 thin tabular crystals -^^ 



(001) (100) with a little 

 (no). They showed a 

 length parallel to the 

 base of 19, 21, and oc- 

 casionally even 24 mm. 

 and a thickness of 2-2.5 

 mm. In slags with 60- 

 70 per cent akermanite, 

 the tabular breadth is 

 commonly 5-10 mm. 



My largest Jayalite 

 crystal (tabular along 

 010) shows a tabular size, 

 25 X39 mm. In fayahte 

 slags with 70-80 per cent 

 fayalite, a tabular length 

 of 25-30 mm is very common. Similar dimensions are also con- 

 stantly met with in manganese-fayahte, (Mn, Fe)2Si04. 



Slags with only 40-50 per cent olivine (Mg,i?)2Si04, (where 

 R = small quantities of Mn,Fe, etc.) show tabular crystals (on 010) 

 about 7-14 mm. Smelting masses consisting of almost pure Mg2Si04 

 I have not examined. 



The freely developed crystals of rhodonite, (Mn,Fe)Si03 or 

 (Mn,Fe) SiOj, in the druses of slags with 65-80 per cent rhodonite, 

 commonly show a length of 12-15 mm., sometimes up to 20 mm. 



Fig. 54. — Large melilite crystals in slag, 

 thirds natural size.) 



(Two- 



