210 HAROLD L. ALLING 
Working with thin sections and crushed fragments of the same 
specimen it was frequently observed that while the thin section 
showed the feldspar frequently twinned the crushed fragments of the 
same material were often untwinned. This applies to plagioclase as 
well as to the potash-soda varieties. In fact some ‘‘adularias” and 
‘‘microclines”” when examined in the form of thin plates (removed 
from the specimen by a knife blade) do not exhibit twinning while 
in thin sections of the same specimen the characteristic twinning 
is at once manifest. An excellent example of this phenomenon is 
the ‘‘microcline” from San Diego County, California. The sug- 
gestion is strong that sufficient pressure to develop twinning was 
present in! the grinding process necessary for the preparation of the 
thin section. If soda orthoclase is metastable at normal tempera- 
tures then it follows theoretically that inversion to soda microcline 
would be hastened by heating. ‘This proved to be the case. The 
San Diego County material was crushed, sieved, and sized, and 
then divided into four samples. One sample was not heated. 
No. 2 was heated one hour in a quartz glass crucible over a special 
Bunsen (Scimatco) burner. No. 3 was heated three hours, and 
No. 4, five hours. It was found that the percentage of the twinned 
fragments increased with the duration of heating. The significance 
of this simple experiment is the raising of the question whether 
thin sections can be relied upon uniformly as a means of proper 
identification of the feldspars. 
The experimental work of Allen and Day’ shows that the 
determination of the melting temperatures of the alkali feldspars 
and the thermal-reaction points indicative of inversions is extremely 
difficult. Working with natural microcline from Mitchell County, 
North Carolina, and employing ‘thermal apparatus .. . . suffi- 
ciently sensitive to detect any unsteadiness of a tenth of a degree 
[centigrade] with certainty, not the slightest trace of an absorption 
or release of heat was found.’’ All such ‘“‘phenomena appeared to 
be effectively veiled by some property [of the substance], presum- 
ably the viscosity.’’ In natural magmas the presence of mineral- 
izers “acting as solvents, keeps the minerals in a fluid condition 
until the temperature is far below that at which they would 
t Allen and Day, Carnegie Institute Pub. 31. 
