78 PEOCEEDIjfGS OF THE GEOLO&ICAL SOCIETY. 



MgO=21, K,O = 10(Total=96*): the composition of a leucitite 

 poor in silica and rather exceptionally rich in magnesia, the result 

 "being that they obtained " apres recuit, un culot crystallin compose de 

 leucite, de peridot, de melilite, et de fer oxydule, c'"est-a-dire, une 

 variete de leucitite a peridot." This interesting result appears to 

 suggest that microcline may, under certain circumstances, be the 

 representative of leucite, notwithstanding their different percentage 

 of silica ; or, to put it otherwise, that the magma which, under 

 certain circumstances, may crystallize as leucite and olivine, with 

 melilite (or with slight differences probably augite), may, under others, 

 form microcline and biotite (potash-iron mica). It is also to be 

 noted that these observers found that the leucite crystallized only at 

 a high temperature ; thus in making artificially a leucotephrite from 

 a mixture representing one part of augite, four of labradorite, and 

 eight of leucite, the leucite crystallized at a " rouge-blanc," the felspar 

 at " rouge-cerise" f. Thus it seems to me unsafe, in the present state 

 of our knowledge, to rely too much on negative evidence afforded us 

 by this one exceptional mineral. 



It appears, then, to me that this attempted classification of igneous 

 rocks into an older and younger series, notwithstanding the autho- 

 rity and a few facts which can be quoted in its favour, not only is 

 in itself improbable, but also is opposed to the general results of 

 investigation, so that its retention will impede far more than it will 

 facilitate progress. 



The order of solidification of the more important rock-constituents 

 presents us with some peculiarities worthy of notice. The separa- 

 tion of iron-oxide takes place at a very early period — probably in 



* This may be modified so as to come nearer to 100. thus : — 

 SiO2=41-0,Al2O3 = 17-42,Fe,O3 = 8-2.MgO=21-52,K,O=10-25,Total=101-39. 

 t The experiments, indeed, of 'M.^L Fouque and Levy (described iu their 

 ' Synthese des Mineraux et des Roches,' a work of tiie highest value to geologists) 

 appear to me to be so suggestive as to the history and relationship of igneous 

 rocks, that L present the results in a tabular form (it will be remembered that 

 the experiments were made by " dry fusion "). 

 (i) Negative results. 



They have failed in obtaining artificially rocks containing free quartz, or- 

 thoclase, albite, white mica, black mica, and hornblende, 

 (ii) Positive results. 



They have succeeded in obtaining artificially andesites and andesitic por- 

 phyrites, labradorites and labradoritic porphj-rites, basalts and labrado- 

 ritic meiaphyres, nephehnites, leucitites, leucott-phrite, and Iherzolite. 

 (iii) Eesults indicating relatiomkip. 



(a) 10 parts of oligoelase with 1 of hornblende produced an augitic andesite : 

 4 parts of microcline with 4*8 of biotite produced the leucitite mentioned 

 above, {h) Microchne with oligoclase, nepheline, and augite produced 

 in each case a glass in which were oligoclase, nepheline, and augite, with- 

 out any trace of a monoclinic felspar, (c) A rock composed of wernerite 

 and hornblende produced a characteristic augitic labradorite with a little 

 melilite. 

 Minerals, however, which they have failed to obtain as constituents in artifi- 

 cially produced rocks, have been separately formed by MM. Fouque and l^exj, 

 and other experimenters, — e. g orthoelase, albite, and' a brown mica, generally 

 after long exposure to a high temperature. Free quartz also has often been pro- 

 duced by the intervention of water. It will be observed from the above tliar, 

 except, perhaps, in the case of Iherzolite, they have chiefly succeeded, as miglit 

 be expected, in producing examples of the less deep-seated igneous rocks. 



