RELATIONS OF VARIOUS FORMS OF SILICA 681 



VARIOUS FORMS OF SILICA AND THEIR MUTUAL RELATIONS 

 BY CLARENCE N. FENNER 



(Abstract) 



The results of laboratory investigation on the conditions of formation of the 

 SiO, minerals, quartz, tridymite, and cristobalite are given, and the bearing 

 which this has on their occurrence in nature is discussed. The inversion point 

 of quartz into tridymite has been determined as 870° + 10° and that of tridy- 

 mite into cristobalite as 1,470° + 10°. These points define the ranges of sta- 

 bility of these minerals, and under proper conditions the appropriate form will 

 always appear, but under certain circumstances tridymite and cristobalite may 

 be formed outside of their range of stability and will persist indefinitely. This 

 has an important bearing on their occurrence in nature. Quartz, on the other 

 hand, does not appear to be formed above 870°, except that great pressure will 

 raise the inversion point to some degree. The thermometric point thus estab- 

 lished places a probable upper limit to the crystallization of quartz from 

 magmas. 



INDEX-ELLIPSOID IN PETROORAPHIC-MICROSOOPIO WORK 

 BY FRED E. WRIGHT 



{Abstract) 



In this paper the importance of presenting the subject of microscopical pe- 

 trography consistently from the viewpoint of the index-ellipsoid as applied to 

 the wave front normals (without reference to the rays) is emphasized. The 

 various optical properties employed — practical petrographic microscope work — 

 can be best described and explained systematically by means of the index- 

 ellipsoid. The use of the so-called "axes of elasticity," or a, b, c, or X, Y, Z 

 in this connection is confusing, and only adds to the difficulties encountered by 

 the observer in mastering the subject. They should accordingly be abandoned, 

 and the French usage of naming the principal axes of the index-ellipsoid a, /3. 5 

 (or rip, m m , ns) adopted. To employ "axes of elasticity" (a/, x' or X', Z') in 

 the expression for extinction angles is not only needless, but less direct, as it 

 introduces entirely new conceptions which experience has shown only tend to 

 bewilder the student. Clear, concise modes of expression and simple methods 

 of attack are as essential in petrology as in other sciences whose development 

 is often directly dependent on the care and attention given by its workers to 

 these factors. 



Discussion 



Dr. E. B. Mathews: Estimating Doctor Wright's suggestions from my own 

 experience as a worker and teacher, I can concur with his judgment that it is 

 helpful to use the Indicatrix and the ellipses desired by the Intersection of its 

 surface by the plane of the microscopic section. This is particularly true for 

 the more mature workers. Although in my teaching during the hist decade 1 

 have used the indicative with varying emphasis and found the practice satis- 

 factory, I have found certain phases of the subject of pet rographical or optical 

 mineralogy which appear to be taught more easily by one o£ the other geo- 



