A2 
The /hncrican Gco/os'ist. 
January, ; 
teristic and the most convenient if its sections could be found 
easily. 
- 
Absolute 
Maximum. 
Extinction at 50" from 
100 : GIG. 
At the left. 
At the right. 
Albite 
6° 
20° 
2q« 
34° 
S2« 
90^ 
+ 4° 
+ 12° 
+ 13° 
+ i7K° 
+ 19" 
+30° 
— 2^" 
OligoclaseAb^An, 
OligoclaseAbsAn, 
Andesine AbjAna 
-13^^ 
— 22>^° 
Labradorite AbiAu 
Anorthite ^ 
-37" 
-79° 
Zone Parallel to 100:010. — This zone furnishes sections 
which are not altogether conclusive, especially for the deter- 
mination of the acid plagioclases, whose extinctions vary too 
widely. It can be said, however, that in this zone the parts of 
the Carlsbad twin extinguish symmetrically. The same is 
true of the lamellae of the albite twinning. All its poles are 
found in the exterior circle of the epures. 
Zone Parallel to the Edge 001:010. — This zone, parallel to 
the two easy cleavages, is in constant application in tjie vario- 
lytes, in arborescent forms of plagioclase, and in crystallites 
formed rapidly. Such are, indeed, elongated parallel to 
001 :oio. The normal alongement of orthoclase, when fibrous, 
is parallel to 001 :oio, as shown by numerous spherulytes of 
the orthophyres and of basic microgranulytes.* 
The spherulytes of the plagioclases do not offer a single 
known exception to the above rule. (Michel-Levy.) 
The extinction numbers belonging to this zone are ranged 
on the meridian which passes through the poles 001 and 010, 
at about 26° below the plane perpendicular to the face 010 
and to the edge 100:010. The following are in its numerical 
properties: (p. 43). 
There might arise uncertainty between the albites and the 
most basic andesines. The smallest extinctions appertain to 
the oligoclases and the acid andesines. From labradorite 
proper the maximum angle of extinction quickly exceeds the 
maximum extinction applicable to the albites. It is worthy of 
*"It is quite exceptional that Williams and Iddings have encoun- 
tered spherulytes of orthoclase whose fibres are elongated parallel to 
