ere cael 
600 General Notes. [June, 
specimens of which one is from Colton, N. Y.,and one from Min- 
eral Hill, Del. Co., Pa. The third-class includes sixteen speci- 
mens, mostly from Scandinavia, and the fourth-class nineteen 
specimens, of which one was from Chateau Richer in Canada. | 
The remaining eleven specimens examined by the author did not 
yield altogether satisfactory results. 
ery interesting facts regarding the relations between the 
monoclinic and triclinic feldspars have been obtained by Dr. Forst- 
ner of Strassburg, through his studies of material from the island 
of Pantellaria.‘ He finds that both the potash (orthoclase) mole- 
cule and the soda (albite) molecule undoubtedly possess a stable 
and unstable modification, the former being for orthoclase in the 
monoclinic, the latter in the triclinic system (microcline), while 
exactly the reverse is the case for albite. It has long been known 
that some varieties of orthoclase, especially sanidine, contain a 
very considerable amount of soda and the existence of the tri- 
clinic form of the potash molecule has been universally acknowl- 
edged since the classic investigations of Des Cloizeaux? Först- 
ner finds on Pantellaria that a monoclinic feldspar containing 2.1 
albite molecules to every molecule or orthoclase occurs as a con- 
stituent of a certain rhyolite (Cala Porticello and Bagno dell’ 
acqua). This he calls “ Natronorthoclase.” It is very similar both 
in composition and structure to that described by Brogger® and 
Miigge* from Norway, and by Klein, from Hohen Hagen, near 
ttingen. Even more common on Pantellaria, as a constitu- 
ent of the augite andesite and pantellarite (a dacite character- 
ized by a peculiar triclinic amphibole called “ Cossyrite®”) is a 
cline. Very remarkable is the ease with which these intermediate 
members—natron-orthoclase and albite-microcline—can be trans- 
ferred by artificial means from one modification to the other. 
Some albite-microcline (Cuddia Mida) shows,even bya temperature 
of 86°-115° C., a disappearance of its twinning lamellz and by 
other optical changes is plainly seen to pass from the triclinic to 
the monoclinic system. The oligoclase-microcline from Mt. Gibele, 
on the other hand, remained unchanged even at 500° C. Most 
feldspars of this series changed, back to their original triclinic 
form when the temperature was again reduced, except when it 
had been raised as high as 500°, in which case the change to the 
monoclinic form was as a rule permanent. The natron-orthoclase 
showed no change while being heated, but on being cooled from 
S aai se hie, VIII, 1883, pp. 125-202; 1x, 1884, pp. 333-352- 
* Comptes rendus, 1876, p. 885. 
3 Die silurisc} Etagen 2 und 3 im Kristi 
Lr 
t, etc, ; Kristiania, 1882, pp. 258- 
262; 293-307. eG: g 
Peg Jahrbuch für Min., ete., 1881, 11, 106. š 
-6 Zeitschrift i 
für Krystallographie, v, 348. 
