l82 REPORTS ON INVESTIGATIONS AND PROJECTS. 



eral were found, and their optical and physical properties carefully estab- 

 lished. One is stable from 1,410° to 2,080°, its melting-point; a second 

 between 675° and 1,410° ; while the third is only found at lower temper- 

 atures. It is interesting to note, however, that no one of these three forms 

 exists in nature on account of the energy with which they are attacked by 

 water and greater or less quantities of lime dissolved out. 



The investigation of one of the components, silica, developed some further 

 interesting information regarding natural formations, and incidentally led to 

 a separate investigation, of which some account has already been given 

 (No. 4 above). Silica was found to crystallize in two forms, quartz and 

 tridymite, of which quartz is one of the commonest natural minerals. Tridy- 

 mite occurs but rarely in nature. Under atmospheric pressure, however, 

 silica can only crystallize as quartz below 800°. Above that temperature 

 tridymite forms, and existing quartz becomes tridymite by slow inversion. 

 Thus one of the most important and one of the commonest of the minerals 

 found in nature must have formed at a relatively low temperature. The 

 difference in density between these two forms of silica is relatively enor- 

 mous — quartz, 2,654; tridymite, 2,318 (more than 13 per cent). The density 

 of the glass is still lower — 2.213. 



(9) Schistosity by crystallization : A qualitative proof. Fred Eugene Wright. Amer. 



Journ. Sci. (4), 22, 224-230. September, 1906. 



The schistose textures of many metamorphic rocks have been ascribed by 

 Van Hise and others to the orienting influence of pressures, with a stress 

 difference acting during the recrystallization of the rock in its new environ- 

 ment, solution taking place along the line of greatest strain and deposition 

 along the line of least resistance and normal to the maximum stress. In 

 such cases the rock cleavage is due to the parallel arrangement of its mineral 

 components in planes perpendicular to the line of greatest stress. 



Conditions of experiment in which crystallization under unequal stresses 

 could take place were effected by using cubes of glass made by rapidly chill- 

 ing various mineral melts, and by heating these slowly to the point at which 

 crystallization began. The glass at that temperature is still so viscous as to 

 be mechanically rigid and capable of supporting unequal pressures. 



Textures similar to those of certain metamorphic rocks were produced in 

 this way, and an experimental confirmation of the theoretical deductions thus 

 obtained. 



(10) Minerals of the composition MgSiOs: A case of tetramorphism. E. T. Allen, 



Fred Eugene Wright, and J. K. Clement. To appear in Amer. Journ. Sci., 

 November, 1906. 



Magnesium metasilicate, when prepared pure, can be made to appear in 

 four different crystal forms, of which two are pyroxenes and two are amphi- 

 boles. The only form which is stable at all temperatures is the monoclinic 



