Wright and Larsen — Quartz as a Geologic Thermometer. 443 



37. Quartz from granite. East slope of El Sobranti, near Corona, 

 California. Gray medium-grained granite consisting essen- 

 tially of quartz, alkali and soda, calcic feldspars and biotite. 

 Spec. 10. (E. S. Larsen, collector.) 



38. Quartz from granite. Near Pevey, Lake Bennett, B. C, 

 Canada. Pale pink medium-grained granite consisting essen- 

 tially of quartz, orthoclase, oligoclase and biotite, and some 

 visible magnetite. Spec. 312. (F. E. Wright, collector.) 



39. Quartz from granite near Meissen, Saxony, Germany. Pink, 

 fresh, medium-grained granite consisting essentially of 

 quartz and feldspar with some biotite. (Charles W. Wright, 

 collector.) 



40. Quartz from granite. Marble Falls, Burnett County, Texas. 

 Medium to coarse-grained pink granite, consisting essentially 



# of quartz, feldspar and biotite. Spec. 38,824, U. S. Nat. 

 Museum. 



41. Quartz from miarolititic pegmatitic cavity in granite. 

 Railway cut near Glacier, Skagway, Alaska. Granite is 

 exceedingly variable in granularity and consists essentially 

 of quartz and feldspar with some biotite. Spec. 282, (F. E. 

 Wright, collector.) 



42. Quartz from granite porphyry, Bassett Mine, Cambourne, 

 Cornwall, England. Granite porphyry with phenocrysts of 

 dihexahedral quartz, orthoclase and tourmaline. (F. E. 

 Wright, collector.) 



43. Quartz from granite porphyry. Yankee Creek, Brooks, Mt. 

 Seward Peninsula, Alaska. Gray granite porphyry with 

 phenocrysts of quartz and orthoclase. (Adolf Knopf, col- 

 lector.) 



44. Quartz from quartz porphyry. Verdugo Protero, ten miles 

 south of Corona, California. Phenocrysts, quartz and feld- 

 spar. Fresh, gray dike rock in andesite. Spec. 190. (E. S. 

 Larsen, collector.) 



regular in boundary lines in the a- than in the /3-form. (4) 

 Plates of originally /3-quartz but now a-quartz by inversion 

 show the effect of the inversion by the shattering which should 

 be most evident on large plates. — Into all these criteria an 

 element of probability enters, and in testing quartz plates, with 

 this end in view, a number of plates should be examined to 

 strengthen the validity of the inferences drawn. 



It was of interest to apply these criteria to actual occurrences 

 of quartz in nature, and for this purpose 44 specimens of quartz 

 and quartz-bearing rocks from different localities were chosen, 

 10 specimens of quartz from veins and geodes, 21 from peg- 

 matites of different types and 13 of granites and granite por- 

 phyries. From each specimen from 3 to 25 plates after the 

 basal pinacoid were cut and polished on both sides. — Each 

 plate was then examined with reference to its circular polariza- 

 tion and the character of its twinning. All plates were etched 



