from the Standpoint of Physical Chemistry. 227 



In the noncrystalline phases the plagioclase gives, at first, 

 an impression of being much more euheclral Q 



than the diopside, but it is almost impossible 

 to find a crystal in which the symmetry is not 

 destroyed by encroachment of diopside, and 

 the extremely irregular growth of plagioclase 

 sketched in fig. 6 \ Slide No. 47) and fig. 8 

 (Slide No. 61) is entirely normal and charac- 

 teristic FlG - 9 Inter ' 



teilSUC. .'■-,. ,n . growth of magne- 



The magnetite is often m such line grains t jt e diopsicle, and 

 that no conclusions can be drawn regarding plagioclase (eutec- 

 its period of growth, but in No. 23 it is found nf d stl N uc l'| re ~ 

 in better developed crystals, which show un- 

 mistakably a simultaneous deposition with plagioclase and 

 diopside. Fig. 9 shows the general relations. 



Paet II. 



The Crystallization of a Magma as affected by the 

 Law of Mass- Action. 



It was pointed out on a preceding page that in order that the 

 crystallization of a magma should follow strictly the laws of 

 eutectiferous solutions the compounds present in the fusion- 

 solution should have no inter-reaction during crystallization, 

 but that simply the freezing-point of each should be depressed 

 by the presence of the others and each should begin to crystal- 

 lize at the appropriate temperature as thus modified. In the 

 Watchung magma it appears that this ideal course was nearly 

 realized as regards the final products of crystallization, and in 

 many other cases this is probably true, especially where the 

 range of temperature from the beginning of crystallization 

 until the final consolidation of the eutectic is not large and 

 where great variations of pressure do not occur. Reference 

 was made, however, to resorbed olivines which appear in 

 certain slides, and resorption-phenomena are of common occur- 

 rence in the study of igneous rocks. Their chief features are 

 explainable by the law of mass-action. 



It is well understood that when substances capable of 

 reaction are brought together in a solution the reaction does 

 not ordinarily proceed to completion, but that in principle 

 every reaction is reversible and the degree to which a reaction 

 will proceed under given conditions of temperature and 

 pressure is dependent primarily upon the concentrations or 

 active masses of the substances participating. 



In solutions of molten silicates it is recognized that we have 

 very little knowledge of the extent to which reactions proceed 

 to attain equilibrium ; but we know that the state of equili- 



