FENNER, THE WATCHUNG BASALT 131 



Period I. Boric acid period. 



Stage 1. Albite, quartz, garnet, amphiboles, specularite, sulphides. 

 Stage 2. Datolite, prehnite, pectolite, amphiboles, specularite, sulphides. 

 Period II. Zeolite period. 



Analcite, chabazite, heulandite, stilbite, natrolite, scolecite, lau- 

 montite, apophyllite, amphiboles, chlorite, specularite, sulphides. 



To these may be added a third period, whose significance will be shown 

 later. 



Period III. Calcite period. 



Thaumasite, calcite, gj'psum, amphiboles, chlorite, specularite, 

 sulphides. 



FIRST PERIOD OF ALTERATION 



In the primary stage of alteration, temperature and pressure were at a 

 maximum. The resulting minerals are those which are characteristic of 

 a rather intense form of hydrothermal metamorphism. They include 

 albite, arfvedsonite and other amphiboles, specularite (specular hema- 

 tite), sulphides, garnet and quartz. 



Alhite 



The feldspar of the unaltered basalt is a medium labradorite of about 

 the composition albite 40, anorthite 60, occurring in hypidiomorphic 

 crystals of lathlike habit, whose largest dimensions seldom exceed 0.5 mm. 

 Among the alteration products, this is replaced by albite, whose size 

 seldom falls below the maximum of the labradorite and ranges from this 

 to a length of 3 mm. or more. 



The process by which the transformation of labradorite into albite has 

 been effected appears most explicable, if the plagioclase feldspars are 

 regarded as members of a continuous series of solid solutions formed of 

 the two molecules albite and anorthite, as appears to have been rendered 

 certain by the researches of A. L. Day and E. T. Allen, to which refer- 

 ence has been made. The ratio in which the two will enter a crystal is 

 dependent upon the composition of the solution in which crystallization 

 is proceeding, and the crystal will continue in equilibrium with the solu- 

 tion only so long as the ratio of albite and anorthite molecules in the 

 latter undergoes no change. 



It is evident that, as the two molecules are chemically distinct, their 

 fields of stability will not coincide and that, under a change of conditions, 

 the crystals may find themselves in contact with a solution in which the 

 anorthite molecule is unstable, while albite is not affected. Under these 

 circumstances, the small amount of albite which dissolves in the liquid 



