278 



PKOCEEUIXGS OF SECTION C. 



intergrowths. It may result in granular aggregations of concerted, 

 adjacent, anhedrons, not intergrown, which corresponds to observa- 

 tions on the textures of igneous rocks ; for many rocks of like 

 composition in some instances exhibit graphic texture, in others evenly 

 granular texture. Accepting graphic intergrowth as evidence of 

 synchronous crystallisation, and of the existence of eutectic proportions 

 in some cases between the several mineral compounds at the moment 

 of ciystallisation, it is to be noted that such intergrowths have been 

 developed in igneous rocks between quartz and potash-feldspar; quartz 

 and sodic feldspar; quartz and biotite ; feldspar and pyroxene ; feldspar 

 and hornblende; feldspar and nephelite; pyroxene and iron oxide 

 (probably magnetite); and between other pairs of minerals. 



The separation of solids, that is, the crystallisation of minerals 

 from rock magmas must be an extremely intricate process, because of 

 the complex character of the solution, the variable and irregular 

 chariges in temperature and pressure consequent on the movements of 

 eruption, the variations in composition due to changes in gaseous 

 components, and the possibility of chemical reaction among the com- 

 ponents with changes of chemical equilibria, as well as the probable 

 supersaturation of the magmas by different components to various 

 degres. This complexity will rmdoubtedly prevent exact statements of 

 ihe relations between composition and texture, but approximations 

 may be made to the proper explanation of some of the most common 

 and characteristic textures, which will render them more intelligible 

 to the student. 



The crystallisation of a substance from solution involves molecular 

 diffusion, and molecular orientation, and these are functions of 

 molecular attraction, composition of the molecular compound, viscosity 

 of liquid, composition of the liquid, temperature, pressure, and time, 

 or rate of changing conditions. The combination of the^se factors in 

 the case of any cooling rock magma results in the rock possessing a 

 certain degree of crystallinity, which may range from a state of 

 com]ilete crystallinity, to the reverse, or complete glassiness. When 

 more or less crystalline, the size of the crystals becomes a feature of 

 consequence. The granularity, or the size of crystals in rocks, has 

 been given a prominent role in most descriptions and classifications 

 of rocks. The shapes of individual ciystals cleaidy give distinctive 

 char-acter to the pattern, or fabric of rocks, and shape is largely a 

 function of crystal structure and physical habit of specific minerals. 

 The recognition of these relationships and their systematic treatment 

 in the description and discussion of igneous rocks will lift the subject 

 out of a mass of confusing, complex detail ; usually treated in an imco- 

 ordinated and meaningless manner. 



Application of principles of molecular diffusion ; of laws relating 

 to solution pressure, or osmotic pressui-e ; of conditions controlling 

 ciystallisation, or the separation of solids from solutions; of conditions 

 affecting the physical character of liquids, or rock magmas ; to the 

 observed variability in the composition of igneous rocks ; and to the 

 known relations between their composition, order of eruption, and 

 mode of occurrence, leads to conceptions of their origin from other 

 magmas, by proce'-ses called by the general designation of differentia- 

 tion. 



