232 The American Geologist. October, i9oo 
Russia, (near St. Petersburg), gave the following measures: 
/jfi (001 ) A (010) =94*^19 '1 94"; ', 94^" 16 ', 94*4 ', averaging 94*^ 
\2'. This crystal has a specific gravity of 2.701; and the ex- 
tinction on ^'(010) is 25°, and on /(ooi) it is p^-^" to 11°. The 
composition is evidently near AbsAm. 
Persistant attempts were made to obtain similar measures 
on material from several other localities, namely, Etna, Italy; 
Ojamo, Finland; lake Champlain, New York, and Wilmington, 
Delaware, but the images from cleavage faces were too con- 
fused to permit any reliable measures, though the values ob- 
tained only confirmed the general conclusion that a remarka- 
ble increase in the angle />^'(ooi)A(oio) occurs as labradorite 
becomes more basic. It is especially regretted that no good 
measures could be obtained with the labradorite from Etna, 
since that was the only material of volcanic origin examined, 
but the images were especially bad, being always faint and 
blurred, or multiple. The material from Ojamo, Finland, gave 
images from pipoi) which, though bright, formed a continu- 
ous series of images of equal brightness extending over sev- 
eral degrees. Such measures as could be obtained were fre- 
quently 90° to 91°, showing that the cleavage /(ooi), instead 
of following the zigzag of the albite twinning, cut irregularly 
across it. 
The exact chemical composition of the material serving for 
previous goniometric measurements is only known in a few 
cases, but the double determinations of this sort, indicate in 
general a rapid increase of the angle /'^'(ooi)A(oio) as the 
feldspar becomes more basic, and an increase beyond the value 
for anorthite for feldspars between Ab^Ans and AbiAui. Ar- 
ranging in order of basicity, the following table results: 
It is unfortunate that so few of those, who have made care- 
ful determinations of the value of angle /»^'(ooi)A(oio), have 
made any chemical analysis of the material used. The few 
analyses it has been possible to find are far from satisfactory; 
thus, it will be seen at once that the high per cent of potassium 
in the first analysis has distinctly decreased the specific grav- 
ity, and probably is the cause of the low silica; the calcium 
oxide is the most characteristic number. The specific gravity 
of the second seems distinctly too high ; the value of the cleav- 
