304 REVIEWS 



quantities ; lime-magmas in small quantity and pure potash-magmas 

 are, under the same conditions, incapable of dissolving alumina in 

 excess. 



4. Supersaturated aluminosilicate magmas, whether of mixed silicates 

 or simple, with the general composition Me0.mAl 2 3 .nSi 2 (Me = 

 K 2 , Na 2 , Ca, n = 2 — 13), throw out all the excess of alumina (over 

 m = 1) in the form of corundum crystals, when magnesia and iron are 

 absent, and n is less than 6 ; in the form of sillimanite (or siliimanite 

 and corundum) when n is greater than 6 ; in the form of spinel (or 

 spinel and corundum) when the magma is rich in magnesia and iron 

 and n is less than 6 ; or in the form of cordierite (or cordierite and 

 spinel) when Mg and Fe are present and n is greater than 6. In the 

 last two cases sillimanite and corundum may also sometimes crystallize 

 out. 



5. The amount of spinel or sillimanite, from magmas rich in mag- 

 nesia or silica, depends wholly on the excess of alumina present. The 

 same is also true of corundum. 



6. The crystallization of corundum and spinel depends, not on the 

 "basicity" of the magma, but only on the ratio of the bases (K 2 0, 

 Na 2 0, CaO) to alumina. In the experiments, corundum crystallized 

 out from magmas varying in silica from o (sodic-aluminate) to 13 

 (Rhyolile). 



7. Rules 4 and 5 are not wholly true for those magmas which con- 

 tain basic non-aluminous silicates like augite and olivine in any con- 

 siderable quantity. 



8. Corundum, spinel, sillimanite, and cordierite crystallize from 

 silicate magmas according to the general laws governing crystallization 

 from solutions. 



In nature, magmas with alumina in excess occur, but are not very 

 common. There are numerous cases of the primary occurrence of 

 corundum, spinel, sillimanite, and cordierite in both plutonic and vol- 

 canic eruptives. The development of these minerals about inclusions 

 and by contact metamorphism in clay slates is well known. These 

 four minerals form a genetic group of close affinity in mode of origin. 

 In the Urals there are numerous orthoclase-corundum rocks classed as 

 pegmatites and syenites. Morozewicz describes fully a new type of 

 great interest to petrographers, and of especial interest in connection 

 with his experiments ; the new rock he names Kyschtymite after the 

 Kyschtym district in the Urals: it consists of a medium-grained mix- 



