234 The American Geologist. October. 1900 
age angle found by Fou'que is only a trifle too high to take its 
place in the regular increasing series; the alkalies of his analy- 
sis indicate AbiAni, but the alumina, silica, and specific gravity 
all point toward AbrAns; on the other hand the determination 
of Scharizer* is a decided exception to the general rule. Vom 
Rath's third analysis is also unsatisfactory; the calcium and 
sodium correspond very closely with Ab^An-s but the gravity is 
much too high. Finally, Holland's calcium determination is 
too low to correspond with the rest of the analysis. 
It may be worth while to note that the little evidence we 
have would indicate that Nordenskjold's feldspar would take 
its place near Abu-Vm in the series, and Abicht and Ricciardi-t 
have published analyses indicating that Marignac-Des Cloi- 
zeaux's material was quite possibly Ab-An.. The only analy- 
sis known of the feldspar from near Kiew gave a result very 
near AbvAns, so that Obermayer's values between 93° 43' and 
93° 52' would take a natural place in the series. The four 
values from the coast of Labrador would also fall nearly in 
their regular places, since many analyses show that the feld- 
spar in that locality is usually very near Ab)x\ni, and may even 
be slightly more acid, while it is rarely found more basic than 
AbTAm. It is therefore evident that the value of the angle 
pg'^{oo\).:\{Q)\<S) in the feldspar AbAni is very near 93" 30'. The 
values found for the various plagioclases can probably be ar- 
ranged as follows: 
Authority 
Locality 
Value 
Composition 
G. Rose 
Labrador 
93«30 ' 
AbiAiii 
Reusch 
Labrador 
93040 ' 
Schrauf 
Labrador 
93<'3G ' 
Marignac-Des Cloizeaax 
Etna 
93°20 ' 
Nordeuskjold 
Ojamo 
93''28 ' 
Voni Rath 
A'^isgerad 
gs'^io ' 
Voin Rath 
VisgeracT 
93°40 ' 
Vom Rath 
Visgerad 
93°43 ' 
Voui Rath 
Visgerad 
Average 
93°45 ' 
93''32'' 
Abj An J 
*His measure may be simply an approximation, as he does not 
say how it was made; it is. indeed, given rather incidentally. 
tAbich: Pogg. Ann. 1840. L. p. 347. 
iRicciardi: Gazz. Chim. Ital. 1881, p. 138. 
