664 GEORGE W. BAIN 



that is, the amount of iron oxide remains constant in the rock. 

 The alumina however accumulates above an amount required 

 to maintain an equiHbrium, and when this condition is attained, 

 some alumina and iron oxides are held by the pegmatitic solutions, 

 which transport them to a zone where lack of equilibrium exists. 

 This happens to occur where the stresses in the rock are such as to 

 permit the formation of garnet and feldspar, that is the stress on 

 the crystal is essentially static load. The available iron and 

 alumina combine with sufficient silica to form almandite. Magnesia 

 and lime also enter into the combination to form grossularite and 

 pyrope molecules. Some of the remaining available alumina 

 combines with the alkalies and lime to form feldspars while a small 

 amount is carried into the limestones to form mica and other 

 aluminous minerals. 



THE DESTRUCTION OF THE GARNET ZONE 



As soon as the garnet feldspar zone has formed, the batholith 

 is free to extend its activity farther into the Hmestone. Lateral 

 stresses caused by mountain folding, igneous activity, and increase 

 in rock volume produced fracturing in the garnet which then 

 became overwhelmed in a zone in which it was unstable. Stress 

 from one direction causes the almandite to break down into chlorite 

 when in the presence of the circulating magmatic waters. The 

 chlorite readily recrystallizes to biotite under the influence of these 

 alkahne solutions. 



For the purposes of studying the alteration of the garnet, the 

 composition of both the mica and the garnet have been recalculated 

 to molecular proportions. (In the case of the garnet it will be 

 noticed that the material used was not entirely freed from the 

 quartz stringers which cut the almandite and which in ordinary 

 Kght look very much Hke it.) 



Garnet Biotite 



SiO. 68s 613 



AI2O3 201.5 219.5 



Fe203 14.8 



FeO 419 19.6 



CaO 42.5 48.7 



MgO 142 228.5 



Na^O 50.3 



K.0 39.3 



