444 Wright and Larsen — Quartz as a Geologic Thermometer. 



uniformly in cold commercial hydrofluoric acid, the time of 

 exposure in every instance being 75 minutes. The etched 

 plates were examined at first both in reflected and transmitted 

 light, but experience soon indicated that the best results were 

 obtained by observing the etched surfaces and figures in 

 transmitted light, the rays being obliquely incident at such an 

 angle as to cast proper lights and shadows across the small etch 

 pits and hills. Even in the process of etching the difference 

 between the high and low forms was often evident. The 

 crystal plates of the a-form were as a rule clear and without 

 fractures, and although on etching all such cracks were promptly 

 discovered and emphasized by the acid, the general appearance 

 of the plates after etching was nevertheless uniform and con- 

 tinuous. Plates of original /3-form, on the other hand, even 

 though clear, before etching, developed after short exposure in 

 the acid numerous cracks, potentially present before, which 

 frequently caused the plate to crumble and break up into 

 smaller grains. As a rule etch figures on such plates were also 

 the least satisfactory. 



The character of the circular polarization of the different 

 plates was ascertained in sodium light, the lenses of the con- 

 densor system of the microscope having been removed and the 

 plates observed with a low-power objective. After etching 

 the plates were re-examined, the disturbing influence of reflec- 

 tions on the etched surfaces being eliminated temporarily by 

 immersing the plates in a liquid of refractive index T554, 

 equal to o> of quartz. 



A detailed discussion of the results assembled in Table 6 

 substantiates in a general way the theoretical inferences. Of 

 vein quartzes, 10 specimens were used, 125 basal plates cut 

 and polished and etched ; of these 49 showed right-handed 

 circular polarization, 50 left-handed,. 16 left- and right-handed 

 intergrowth of fairly regular outlines ; 50 plates were not 

 twinned, while 63 were twinned, the outlines of the twinned 

 areas being in general regular and indicative of hexagonal 

 symmetry. Practically all of these plates were free from frac- 

 ture cracks of any importance. Twenty-one specimens of peg- 

 matite were examined and found to fall naturally into two 

 groups. Nos. 11 to 20 were taken from large masses of quartz in 

 pegmatite dikes and masses and in certain cases were definitely 

 stated by the field relations to be the last portions of the 

 pegmatite to crystallize out. In behavior they resemble vein 

 quartz and have in all probability never been heated above the 

 inversion temperature. From these specimens 11 to 20, 102 

 basal plates were cut and polished and etched ; of these 35 

 were right-handed in circular polarization, 35 left-handed, and 

 20 intergrowths of right- and left-handed individuals, the out- 

 lines of the different intergrowths being in general regular and 



