J. D. Dana on the Feldspar group of minerals, 401 
Anorthite (R34 3A0)*Si% SipO.](GR+36Al). 
inalag (ZR? +241)? Si28+35Si SipOs|(2(R, Rs)-+22Al).+48i0? 
desi : SES Lge : 
Oe Saco, ef (ahs .2&1? Si? +35: Si] O.1(2(R, Ra)-+ $8A)).4+8i02 
Oligoclase (GR? 4.341)?Si8 +3351 Si] O.](2(R,R2)+$2Al).+148i0? 
— ; (Re +241)2S*+ 6$i Sif O.] (PR, 29Al), +2810" 
According to either of these two methods, A or B, any other 
proportions of silica exceeding that of the Unisilicate may be 
expressed in the same manner as the above. 
The last of the formulas under A, if expanded, becomes,— 
Old (k?)2 Sis + skis + 48irsie 
New SiJO.[(Ra)2 + 3[SiJO.[SAl,] + 4[SifO.fySie] 
in which the last member consists of silica alone; and thus the 
formula approaches the corresponding one under B, yet with 
this important difference, that the silica in this member is in 
the former in two states, a basic and an acid. 
5. The excess of silica above the amount required for the 
Unisilicate increases with the increase of the alkalies, or the 
Species are, for the average results of analyses, as follows : 
on-alk. : Alk. Non-alk. : Alk. 
Labradorite ee Oligoclase 1:2 
Hyalophane 1g: 1 Orthoclase all alkali. 
Andesine is} Albite all alkali. 
| _ Thus the above statement is fully sustained by the average re- 
{ Tesults of the analyses. Looking to special analyses, it will be 
_ found that in general those Labradorites which contain less than 
normal proportion of silica (and so approach Anorthite) are 
those which contain most lime in proportion to the alkali ; and 
those which give the oxygen ratio 1:3: 65, 1:3: 7 are those 
which contain east lime relatively to the amount of alkali; and 
80 for the ratios between 1:3:8 and 1:3: 9, and others be- 
_ tween 1:3:9 and 1:3:12. There are exceptions to this 
Statement, as should be expected when we consider the diver- 
sity of causes (see p. 309) which may have aided in ——- 
a these intermediate ratios. But the cases in accordance are sul- 
ficient ‘to sustain it. cae : 
_ It would seem, therefore, that this excess of silica is depen- 
ine or perissad nature 
: dent in part, +, if not wholly, on the alkaline or 
of the base, ieee 
SE ee Se ars Na 
