STUDY OF IGNEOUS ROCKS. 267 



often of much value qualitatively, but sometimes misleading. Other 

 recent and valuable qualitative work in this field has been done by 

 Morozewicz, while earlier work is represented by the classic I'esearches 

 of Daubree, Fouque, Michel Levy, and others. 



The most obvious result of the earlier efforts was the demonstra- 

 tion of the adequacy of fusion and gradual cooling at ordinary atmos- 

 pheric pressures to bring about the ciystallisation of many minerals 

 found in igneous I'ocks ; and the necessity of some catalytic agency to 

 promote the crystallisation of other minerals common to these rocks. 

 Such actions were ascribed to " mineralising agents " or "" crystal- 

 lisers," assumed to be in most instances dissolved gases, chiefly H2O. 

 (Jne of the most significant facts brought to light bv the researches of 

 Day and his colleagues is the new conception of high viscosity found 

 in alkali-feldspars and quartz. Viscosities so great at temperatures 

 near the transition point of liquid to crystal phase as to be indis- 

 tinguishable within the two phases. That is, the viscosities of the 

 amorphous glass, and of the crystallised mineral, are so nearly 

 identical that the two phases of the substance react alike toward 

 mechanical stress. Molecular mobility is so slight that readjustment 

 from crystalline arrangement to the homogeneous chaos of liquid 

 molecules is accomplished with such extreme slowness that the time 

 of ordinaiy laboratory obsei-vation is not sufficient for its detection. 

 However, the time available for ordinary '"geological" processes, so 

 called, is sufficient, as shown by the devitrification of volcanic glasses 

 composed of these constituents — ancient rhyolitic obsidians. The 

 function of a catalytic agent, as a gas dissolved in such a viscous liquid, 

 io obvious. The viscosity is reduced and molecular mobility increased. 

 If the transition is towards the liquid phase, solubility of the crystal is 

 increased. If it is toward the crystal phase, the rate at which ciystal- 

 lographic molecular arrangement is accomplished is increased. The 

 dissolved gas becomes a " ciystalliser," or "mineralising agent". 

 Other substances, such as mineral compounds, yielding less viscous 

 liquids than those of the alkali-felspars and quartz, when dissolved in- 

 the more viscous liquids reduce their viscosity in the same manner, 

 though not to the same extent, as dissolved gases. They must behave 

 catalytically toward crystallisation as gases do. Their behaviour in 

 this respect has not been generally recognised, though the function of 

 certain liquid compounds as fiuxes or as "' mineralising agents " is 

 well known. 



Thus the improvement in methods of physical research is steadily 

 enlarging the field of petrological investigation, and the advancement 

 in the knowledge of physicochemical laws is furnishing the investigator 

 with new tools for the woi'k, and m.ore efficient means for attacking 

 the problems of igneous rocks. Foremost in the ranks of those who 

 have attempted the application of modern conceptions of physical 

 chemistry to the elucidation of the phenomena of texture and mineral 

 composition, and of the genetic relationships of igneous rocks, is Vogt, 

 whose earlier studies of furnace slags opened the way for the explana- 

 tion of many analogous phenomena in the more refractoiy, volcanic 

 lavas. Chief among these are: the apparent order of crystallisation 

 of different minerals in slags as indicated by their shapes and relations 

 to one another as inclusions; and the relation between these orders 

 and the composition of the mixture from which they crystallised. 



