USEFUL METALS 



221 



secondary minerals hut their discussion belongs with the non- 

 metallics, 



Character of the Ore Bodies. Corundum occurs as oval or 

 lenticular ore bodies near the margin of pcridotites or norites. 

 They difTcr in many respects from those formed \,y the thermal 

 metamorphism of sediment-. There is usually a complete ab- 

 sence of such metamorphic minerals as cyanite, which plays an 

 important part in the corundum deposits of Siam where the ore 

 is a product of metamorphism. In the case of the peridotites 

 the alumina was dissolved in the original magma at the time of 



FIG. 117. View of Bauxite bank, Rock Run, Alabama. (By permission of 

 . the Macmillan Company, from Ries' Economic Geology.) 



its intrusion into the country rock and then separated forming 

 lens-shaped masses at an early stage as the magma began to cool. 



Alumina is also soluble to some extent in molten magnesium sili- 

 cates, and if the magma has no excess of magnesium all the 

 aluminum will crystallize as corundum, but if there be a slight 

 excess of magnesium over that required for the magnesium sili- 

 cates then spinel will be formed. 



Corundum occurs as a contact deposit between some intrusive 

 and limestones or shale. In Massachusetts at Chester corundum 

 and emery occur in a large vein consisting mainly of emery, 



