Journal of Applied Microscopy. 483 



The charged crucible was first placed in one of the canals and brought to a 

 red heat, then moved into the full heat of the furnace. When the charge was 

 melted the crucible was moved back nearer the walls and left there for several 

 days, and then moved into the still cooler canal. 



The whole operation varied from one week to two months, and the temper- 

 ture at the maximum heat of the furnace was about 1600° C, and near the walls 

 about 1200°, and perhaps 600° in the canals. During casting times these 

 temperatures were very much lowered. 



The conclusions of Lagoriof that a magma is a solution of different but 

 definite silicates in undetermined proportions and that these solutions in crystal- 

 lizing are governed by the same laws as an aqueous solution of several salts, 

 were taken as the basis of the investigation. 



Conditions governing the fo?'mation of Coriindi/fn, Spinel, Sillimanite, and 

 Cordierite in Silicate Magmas. — The result of a large number of meltings under 

 different conditions, and of a careful chemical and optical analysis of the 

 products, lead to the following very definite conclusions. 



After defining silicate magmas saturated with alumina as those in which the 

 ratio of other bases to the alumina is unity, the conclusions are, in brief : 



1. That such saturated solutions can at higher temperature dissolve alumina 

 and become supersaturated. 



2. That soda alumina silicates dissolve alumina in large amounts, lime (or 

 magnesia) alumina silicates dissolve alumina in small amounts, and pure potash 

 alumina silicates do not dissolve alumina at all. 



3. The formula of a silicate magma supersaturated with alumina may be 

 written : MeO, m AlgOg, n Si02, in which Me ^ Nag, Ca, Kj, etc. 



Upon crystallization of such a magma the entire excess of alumina (;// — 1 parts) 

 separates in the form of : A. Magmas with less than y^. per cent, of MgO + 

 FeO: 



{a) Corundum if n is less than 6. 



{p) Sillimanite, or sillimanite and corundum, if n is greater than 6. 

 B. Magmas with over ^< per cent. MgO -|- FeO : 



Spinel, or spinel and corundum, if ;/ if less than 6. 



(^/) Cordierite, or spinel and cordierite, if ;/ is greater than 6. 



4. The amount of spinel, corundum, or sillimanite depends upon and is 

 strictly proportionate to the degree of supersaturation with alumina. 



5. The crystallization does not depend upon the basicity of the magma, for ;/ 

 may vary from 1 to 13, but only upon the ratio of the other bases to the alumina, 

 and separation can take place whenever there is more than one part of alumina 

 to one part of other bases. 



Evidently the sequence of crystallization is not based upon fusibility, but 

 upon supersaturation ; crystals of a substance can only separate from a solution 

 supersaturated with the substance. 



As a result of examination of pyroxene crystals obtained, the author concludes : 

 The pyroxenes separated from silicate magmas, are isomorphous mixtures of Me 



tUeberdie Natur der Glasbasis, sowie der Krystallisationsvorgange im Eniptiven Magma. 



