Vol. 5] 



Louderback. — Benitoite. 



367 



The rocks of the inclusion have all suffered more or less altera- 

 tion affecting throughout the feldspathic constituents and char- 

 acterized by the production of newly formed albite and, as the 

 alteration proceeds, by the production of soda-bearing amphiboles 

 from the ferro-magnesian constituents. Both the igneous and 

 sedimentary types of the inclusion share in the alteration, which is 

 remarkable in that the central portions are more strongly affected 

 than the periphery. This can be explained by considering that 

 emanations from below passed upward through the central por- 

 tions of the rock-mass, and diffused out into the peripheral por- 

 tions. Open channels may have existed in this central avenue, for 

 it must have been an easier pathway than either the peripheral 

 portions or the rock beyond. But if so the conditions at that time 

 were not favorable to deposition along such channels. Further- 

 more, the extent and character of the alterations cannot be ac- 

 counted for by such diffusion as often takes place in the formation 

 of veins. One would expect the pressure to have been rather high 

 and the temperature of the rocks undergoing alteration at least 

 a few hundred degrees above the normal to favor the wide 

 diffusion and production of albite and the amphiboles. 



At least towards the end of this period of metamorphism we 

 have the leaching of feldspathic material along portions of the 

 central tract that produces the porous amphibole rock and the 

 drusy spaces lined with amphibole needles in which the sodium- 

 aluminum silicates are later deposited. This may be the same 

 solution that, diffusing farther into the rocks, causes the re- 

 crystallization. 



The first deposition along the zone of veination is probably 

 that of albite, which is separated out in crystals over and about the 

 amphibole druses and is accompanied at least in one place by 

 aegyrine. Only a comparatively slight change is necessary to 

 change a solution from a condition where it will slowly take up 

 albite to one in which it will slowly deposit it in small quantity. 



Before the natrolite deposition takes place important changes 

 must have been brought about. The action is one of active 

 deposition and is limited to the vein except where the wall-rock 

 is porous. It is then impregnated for a short depth with natrolite 

 and sealed, and the deposition in the vein continues with no 



