10S 
FOURTEENTH REPORT. 
lossiliferous strata may be found and fossils were reported by Walcott, 
at the Christmas meeting- 1911, of the G. S. A., in probably Pre-Kewee- 
nawan rocks north of Lake Superior. 
It may be possible in the marine Cambrian, not far from the 
Keweenawan region, to find such signs of contemporaneous volcanic 
activity as to be closelv correlated with the Keweenawan. Dalv thinks 
there are such. 
SOME INTERESTING CHANGES IN THE OPTICAL PROPERTIES 
OF CRYSTALS WITH TEMPERATURE. 
E. II. KRAUS AND L. J. YOUNGS. 
By using the ordinary metal air-bath, which accompanies the Fuess 
axial angle apparatus, as an oil bath, the changes in the apparent angle 
of the optic axis of gypsum can be easily determined at various tempera- 
tures up to 132.5° C., when the section becomes opaque. The average of 
the determinations of the temperature of uniaxiality in oil is 89.67° C. 
The temperature for uniaxial conditions in air was found to be some- 
what higher, namely 91.6° C. These observations were made with 
sodium light. These temperatures are all lower than those given by 
Des Cloizeaux and Tutton, but show that the early determination in 1826 
by Mitscherlich ol 91.8° C. is substantially correct. Very recently Hutch- 
inson and Tutton have- given the temperature of uniaxiality in water as 
91 C. for sodium light. The changes in the apparent angles in oil 
and air, the unequal changes in the position of the two axes, and the 
acute bisectrix, and the relation between the apparent angle in oil and 
the real angle where shown by a series of curves. (The paper is to be 
published in detail in German in the Neues Jalirbuch fur Mineralogie, 
Geo log ie. etc.) 
