494 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



become gradually more acid, the outermost zone in crystals 

 whose growth was not much interfered with being an 

 oliogoclase (approximately Ab 6 An , ). At occasional points 

 on the periphery of crystals where the final mother liquor 

 consolidated, the outermost zone seems to be nearly pure 

 albite. The decomposition of the crystals renders their 

 zoning evident in ordinary light. The most abundant 

 product of their alteration is undoubtedly analcite, but 

 where this has not been formed, the outer zone, chiefly of 

 oliogoclase, and perhaps often andesine, is marked out by 

 the familiar dusty deposit of kaolin, whilst the kernels of 

 labradorite are clear and glassy. Large patches of the 

 crystals are replaced by analcite; and the crystals, both 

 inner and outer zones, are traversed by many irregular 

 widened cracks, filled sometimes with serpentine and some- 

 times with chlorite, but most often with analcite. 



Whilst the felspar of the great mass of the rock has the 

 composition and characters just described, there occur in 

 segregation veins and in the rock bordering upon them, 

 alkali felspars of two kinds. In the aplitic veins (consisting 

 of essexo-aplite) occurs pure albite in idiomorphic crystals, 

 tabular on 010, twinned on the carlsbad and albite laws, 

 and much altered to kaolin, but not at all to analcite. The 

 other alkali felspar is a soda-orthoclase. It occurs in the 

 aplite, side by side with the albite crystals, and often 

 enveloping them, and also in the rock lying on either side 



volume. The outer half of each crystal varies continuously from the 

 composition of the core to pure albite, and its average composition is 

 taken to be roughly the arithmetic mean in molecular proportions between 

 the compositions of these two extremes. Then if Ab^Any be the compo- 

 sition of the core, the composition of the outer part will be Ah( x +y) + 

 AbzAnt/ or Ab(2*+y) An y . Assuming that there are approximately equal 

 numbers of molecules in equal volumes, the composition of the whole 

 crystal would be Ab(2a+<y) An y ± 2(Aba; A%) and this is equal to the 

 known average composition Ab 4 An 3 , whence we have the equations 



4ai 4- v = 4 x — f x = 

 o * , or 7, or . 



3 y = 3' y = V y = 4. 



