94 REPORTS ON INVESTIGATIONS AND PROJECTS. 



(5) Some mineral relations from the laboratory viewpoint. Arthur L. Day. Bull. Geol. 

 Soc. Amer., 21, 147. 1910. 



Until recently, petrologists have confined their attention largely to the 

 collection and examination of the field and microscopic evidence bearing on 

 the rocks and their mode of occurrence, while the more precise quantitative 

 methods of attack have been slower in development and are only now begin- 

 ning to be considered seriously. This is especially true of experimental evi- 

 dence, and one of the chief purposes of this paper is to invite the considera- 

 tion of geologists, or more particularly, of petrologists, to certain phases of 

 the problem of rock formation as they begin to appear from the viewpoint 

 of the laboratory investigator. 



The laboratory student, for example, proceeding from the physical stand- 

 point, recognizes melting or crystallization as a "change of state" involving, 

 according to accepted molecular theory, a complete change in the molecular 

 structure of the substance. Such a change will carry with it various visible 

 evidences of its occurrence, as, for example, the appearance or disappearance 

 of crystal structure ; a change of density ; a change in the electrical conduc- 

 tivity; in the case of melting, a more or less sudden appearance of fluidity 

 causing it to take the shape of the containing vessel ; a change in the specific 

 heat — in a word, there appears a more or less conspicuous discontinuity in 

 all its physical properties. If we would therefore determine the crystallizing 

 or the melting temperatures of a great body of substances, it is not merely 

 necessary to be able to measure temperatures accurately and conveniently, 

 but also to obtain sufficient knowledge of the individuality of the substances 

 under investigation to enable us to be quite sure that the method employed 

 for detecting the change of state when it occurs is an appropriate one for 

 each particular substance. Any method which allows considerable latitude 

 to the judgment of the observer will fail, as it has done heretofore, to yield 

 uniform and therefore trustworthy results on which to base serious geologi- 

 cal conclusions. 



The effect of small quantities of by-mixtures in lowering formation tem- 

 peratures and the importance of the volatile constituents which have partici- 

 pated actively during all the earlier stages of rock formation but of which 

 only significant traces are now found, can be determined only in the labo- 

 ratory. It is through the study of the volatile constituents also that one of 

 the chief effects of pressure as a controlling force in rock formation has 

 been brought to light ; the pressure serves to retain these fugitive ingredients. 



Laboratory experience, and in particular the study of simple two-compo- 

 nent mineral solutions from the viewpoint of physical chemistry, throws 

 much light on the order of crystallization of minerals from the magma. 

 Physical chemistry distinguishes but two general modes of formation: (1) 

 the eutectic series, in which the excess component first appears separately, 

 followed by a mixture of both in fixed proportions, called the eutectic; (2) 

 the isomorphous series, in which the two components always appear together, 

 the first crystals to form being usually considerably richer in the higher 

 melting component, followed in gradually changing proportions by mixtures 

 of lower melting-temperature. When the number of component minerals is 

 greater than two, the situation of course becomes more complicated, but the 

 two modes of formation above indicated are perfectly general and cover the 

 whole ground. The establishment of the fact that such generalizations of 

 physical chemistry find application to mineral mixtures is of particular inter- 

 est from the viewpoint of the older petrology, for it appears at once that a 

 particular mineral, magnetite for example, does not necessarily take prece- 



