clarke.] FELDSPARS AND SCAPOLITES. 31 
Upon this basis leucite is clearly reduced to the uniform type of the 
minerals to which it is related crystallographically, and also of those 
into which it alters; but the formula proposed can not be regarded as 
final. It is offered here only as a first approximation toward answer- 
ing the questions which are suggested, and is therefore subject to 
modification in the future. 
Closely allied to leucite is another isometric mineral, analcite, which, 
empirically, has the composition ADTaSi 2 O c ,H 2 G. This species alters, 
like leucite, into feldspar, and an alteration into prehnite is also 
recorded.* Furthermore, Lemberg has shown, in the papers already 
cited, that leucite, by the action of soda solutions, may be transformed 
into analcite, and that analcite, by similar treatment with potash, yields 
leucite again. With these facts in view, analcite may be Avritten 
Si0 4 = Na 2 
/ >Al-Si() 4 =H(AlGH) 
Al-Si 3 08=Ha2 3H 2 
\si 3 8 E=Al 
one-fourth of the water being regarded as constitutional. This last 
supposition, however, needs to be checked by careful experiments upon 
dehydration, for the data now on record are not absolutely conclusive.! 
A change in this detail would modify the formula given, but would not 
affect its general type. 
Still another isometric mineral, pollucite, should possibly be consid- 
ered here. Two empirical formulae deserve to be regarded : one, based 
upon analyses of the Elba mineral, and given by Groth, H 2 Cs 2 Al 2 Si 5 15 ; 
the other, deduced by Wells | for the Maine pollucite, H 2 Gs 4 Al 4 Si 9 G 27 . 
Both formulae give metasilicate ratios, but both are reducible to the 
type of leucite and analcite, thus: 
EJha. Maine. 
Si 3 O 8 =0s 2 /Si 3 8 _ Os,ir 
\ Si30 _ A1 \Si 3 3 _ ;(A10), 
Neither formula can be regarded as conclusive, and their only value 
here is to show that pollucite is not necessarily a metasilicate, and that 
it can be represented as similar in constitution to the minerals with 
which it is morphologically allied. Wells's analyses are probably the 
better; but the formula drawn from the Elba pollucite indicates the 
closer analogy to leucite and analcite. 
Although kaolin, mineralogically, is not a member of the feldspar 
group, it is properly discussable here as a derivative. The formula 
assigned to it in the foregoing pages is not unimpeachable, but it sug- 
gests its relations to the feldspars and micas, and also represents the 
* Dana's System of Mineralogy, Oth ed., p. 598. 
t Compare Herscli, Det Wasaergehalt der Zeolithe, Ziirich, 1887. 
J Am. Jour. Sci. (3) XLI, p. '213, 1891. 
