118 A SWALES yEW YORK ACADEMY OF .SCIENCES 



been taken completely into solution and had reacted with lime and silica 

 to form datolite, no further supply was available, and it became an almost 

 negligible factor. It appears probable from observations as to the period 

 of stability of datolite which will be noted later, that from the beginning 

 of alteration up to the time of formation of zeolites (a period in which 

 garnet, amphibole, prehnite and pectolite were deposited, but in which 

 feldspathoid compounds are small in amount or entirely lacking), con- 

 centration should be considered an independent variable, but in the later 

 periods, in which zeolites are prominent, it should be considered to be 

 nearly or altogether a fimction of temperature. To a minor degree, the 

 presence of ferromagnesian silicates acted in a manner similar to boric 

 acid in giving to the factor concentration an independent character, for 

 the presence in solution of any compound having an ion in common with 

 the feldspathoid series would affect their solubility, independently of 

 temperature. The chief eifect of these also is believed to have been in 

 the first stages of alteration and to have become more nearly negligible 

 during the later stages. The numerical value of F may be considered to 

 vary from 2 in earlier stages to 1 in later. 



Solving the equation for P, it appears that during the earlier period 

 the greatest possible number of phases which could exist in equilibrium 

 was five. The liquid and gaseous phases may be neglected, leaving three 

 solid phases. During the later period, it seems at first that four solid 

 phases in equilibrium would be a possibility, but a number of limiting 

 conditions may be applied.^^ 



In all the feldspathoid combinations, which alone are now to be con- 

 sidered, the proportion ISTagO -\- CaO : ALOg : : 1 : 1 obtains. This de- 

 creases by one the number of phases possible and limits it to three. It 

 appears also that with certain members of the series, there is merely a 

 difi^erence in the ratio of SiOj or HoO to the other components. With 

 analcite and natrolite, for instance, the only essential difference appears 

 to be in the ratio of SiO, : analcite = ]Sra20Al2032Si022H20 ; natrolite 

 = ]Sra20Al2033Si022HoO. These two species, therefore, cannot coexist in 

 equilibrium under any change of conditions. ^^ "With heulandite and stil- 

 bite, the essential factor of difference appears to be the ratio of combined 

 water. 



With chabazite, the constitution is very complex, and it is doubtful 

 what molecules could be removed without causing the crystals to break 

 up. It is apparent, however, that one of the molecules cannot coexist 



32 W. D. Bancroft : Op. cit., p. 234. 



^ In the application of the phase rule, there is no concern with the absolute composi- 

 tion of the molecule. Only the ratios are of importance. 



