Wright and Lav sen — Quartz as a Geologic Thermometer. 445 



hexagonal in character ; 30 were not twinned, while 58 were 

 twinned and the outlines of the twinned areas were on the 

 whole regular and indicative of hexagonal symmetry. The 

 plates were as a rule 'clear and free from fracture cracks. — 

 The remaining eleven specimens of pegmatite were from 

 pegmatites showing graphic intergrowths of quartz and feld- 

 spar or coarse-grained aggregates of these minerals. The 

 quartzes from these pegmatites accord in their behavior with 

 original /3-quartz later inverted to a-quartz. Of these 128 

 basal plates were cut and polished and etched ; 37 of which 

 were of right-handed quartz, 81 of left-handed and 3 inter- 

 growths of right- and left-handed individuals, the outlines of 

 the intergrown areas being irregular; 31 plates were not 

 twinned while 53 were twinned, the twinned areas being small 

 and irregular in outline. The plates were in general much frac- 

 tured and shattered. — Thirteen specimens of granites, granite 

 gneisses and porphyries were examined and of these 89 plates 

 cut, polished and etched ; 34 plates showed right-handed 

 rotatory polarization, 46 were left-handed while 3 were inter- 

 growths of right- and left-handed individuals, the outline of 

 the latter being on the whole fairly regular ; 11 plates were 

 not twinned, while 53 were intricately twinned, the boundaries 

 of the twinned patches being small and irregular and without 

 reference, so far as could be observed, to hexagonal symmetry. 

 These plates were without exception small and traversed by 

 fracture cracks which rendered it difficult to obtain satisfac- 

 tory results from etching. The average diameter of the 

 surface of the plates of vein quartz was 5 mm ; of the quartz 

 from the vein pegmatites, 7 mm ; of the quartz from the granite 

 pegmatites, 2 mm ; of the granite quartzes, 2 mm . 



Summarizing these data still further, it may be stated that 

 the quartzes from veins and geodes and certain vein pegmatites 

 are in general clear and free from intricate fracture-cracks and 

 show frequent regular intergrowths of right- and left-handed 

 quartzes ; they are also frequently twinned after the unit prism 

 and the outline of the twinned areas is usually regular and 

 hexagonal in aspect. The quartzes from graphic and granite 

 pegmatites, granites and porphyries, on the other hand, are 

 smaller in size, frequently fractured and cracked in an intricate 

 manner; they show rarely intergrowths of right- and left- 

 handed individuals and the outlines of such intergrowths may 

 or may not be regular. They are as a rule intricately twinned 

 and the twinned areas are usually small and irregular and bear 

 no apparent relation in outline to the hexagonal symmetry. — The 

 observed characteristics of the first group of quartzes are those 

 deduced theoretically for low temperature a-quartzes, while the 

 features recorded for the second group are essentially those 



