;134 Revieics — J. H. L. Vogt — Rock-Magmas as Solutions. 



Thus for olivine (melting-point 1400°), diopside (1225°), and 

 akermanite (1175°), the eutectic mixtures of the several pairs give 

 melting-points — olivine-diopside 1125°, olivine-akermanite 1030°, 

 diopside-akermanite 1035°; while for the ternary eutectic olivine- 

 diopside-akermanite the melting-point is well below 1000°. 



The relations of minerals, such as quartz and the alkali-felspars, 

 which are absent from slags, cannot, of course, be investigated by 

 such methods ; but it may be possible to obtain information in other 

 ways. Vogt confirms Teall's view as to the significance of the 

 micropegmatite intergrowth, and finds the eutectic proportions for 

 felspar and quartz to be about 74 : 26. The figures seem to be very 

 nearly the same in plutonic as in volcanic rocks ; and theoretical 

 considerations also lead to the conclusion that eutectic ratios are not 

 greatly influenced by pressure. 



An interesting branch of the inquiry is that relating to ' mixed 

 crystals ' belonging to isomorphous groups, in which the author 

 applies the theoretical conclusions of Bakhuis Koozeboom. For 

 mixtures of two isomorphous compounds the melting-point curve 

 and the freezing-point curve are different. The crystals accordingly 

 differ in composition from the magma in which they form, and they 

 change their composition (or crystals of continually changing com- 

 position are deposited) as the process goes on. Several different 

 types are distinguished. The author discusses especiall}^ the case of 

 the plagioclase group, and shows why the first-formed crystals have 

 a higher ratio An : Ab than the magma from which they are formed. 

 He discusses also the interrupted series (imperfectly isomorphous or 

 perhaps isodimorphous) of orthoclase-plagioclase, in which the only 

 members which can exist are those near one or the other end of the 

 series. Here there is a eutectic ratio 40 : 60. If the ratio 

 Or : Ab + An is greater than this, orthoclase (containing more or 

 less Ab and An) will first form ; if the ratio is less, a plagioclase 

 (with more or less Or) will form. If the magma eventually reaches 

 the eutectic composition, orthoclase and plagioclase will separate out 

 simultaneously ; and the perthitic intergrowths are of this nature, 

 with constant composition as regards the ratio of orthoclase to 

 plagioclase. This part of the subject is very beautifully worked out 

 from Lagorio's analyses of porphyritic felspars and the magmas from 

 which they crystallized. 



As regards the principal constituents of igneous rocks Professor 

 Vogt's work seems to give a final answer to the vexed question of 

 * order of consolidation.' There remain the minor accessory 

 minerals, which in general crystallize at a very early stage, although 

 they are present only in small amount. It seems impossible to 

 account for this behaviour by reference to eutectic proportions. 

 Our author explains it by Nernst's law of the reduction of solubility 

 by a common ion. This, which is a corollary of the law of mass 

 action, tells us that spinel, for example, which has one ion (Mg) ia 

 common with other constituents of the magma, has its solubility 

 thereby reduced, and consequently crystallizes earlier. It would be 

 satisfactory to know more exactly in what measure this principle is 



