from the Standpoint of Physical Chemistry. 233 



In all cases the olivine shows unmistakably that corrosion 

 had attacked it and that it was in process of resorption by the 

 magma, from which plagioclase, diopside, and magnetite were 

 beginning to crystallize. At some point in the history of the 

 magma the olivine which was first deposited became unstable 

 and would undoubtedly have gone completely into solution if 

 sufficient time had been allowed. It is noticeable that in the 

 later crystallization olivine did not again appear as one of the 

 products. It must follow, therefore, that the olivine molecule 

 which was absorbed underwent some change within the solu- 

 tion by which it entered into new combinations. 



Considered from the standpoint of the principles previously 

 developed, the most probable explanation of the resorption of 

 the olivine appears to be the following : Before extrusion the 

 magma had cooled sufficiently so that crystallization had begun. 

 Inasmuch as most rock-forming minerals occupy less volume 

 in the solid than in the fused condition, the great pressure to 

 which the magma was subjected aided crystallization, so that 

 it was initiated at a somewhat higher temperature than would 

 be the case under less load. The effect upon the crystalliza- 

 tion of a given mineral would vary inversely as the heat of 

 solution and directly as the difference in volume between the 

 two states. Pressure may also have had an appreciable effect 

 upon the direction of reaction within the fusion-solution. 



Olivine was the chief mineral deposited under these con- 

 ditions but there are indications that a little plagioclase had 

 also begun to crystallize out. (In slide No. 47 a large plagio- 

 clase crystal shows some indication of zonal structure, most 

 probably developed during this period.) 



When extrusion of the magma occurred it was attended by 

 great changes in all the physical conditions. 



Diminution of load would be a factor of prime importance 

 in causing the minerals which had crystallized out to be 

 resorbed. The loss by volatilization 'of aqueous and other 

 vapors also affected the combinations existing within the solu- 

 tion. It can hardly be supposed that the vapors evolved 

 existed in the solution entirely in the combinations in which 

 they were given off. Undoubtedly a portion of the vaporized 

 material was combined with the nonvolatile portion and its 

 removal from the system necessitated more or less readjust- 

 ments of equilibrium throughout. 



Under these circumstances it is not surprising that most of 

 the olivine which had been deposited was taken up by the 

 magma. Still if the olivine molecule continued its existence 

 in the re-fused mixture it would be redeposited when the appro- 

 priate temperature was reached under the new conditions, but 

 it appears that the lower temperature to which the initiation 



Am. Jour Sci.— Fourth Series, Yol. XXIX, No. 171.— March, 1910. 

 16 



