50 DR. J. W. EVANS ON A MONCHIQtllTE FROM [Feb. I9OI, 



It is evident that the large green augites with definite crystal-out- 

 lines separated out before the brovp^n hornblendes, for the former 

 have in some cases served as a nucleus for the formation of the 

 latter in crystalline continuity ; and this is the normal order of 

 succession where the hornblende contains alkali. The green horn- 

 blende must have preceded the small granular augites that so 

 often occur in glomeroporphyritic groups, for these aggregates seem 

 in some cases at least to be pseudomorphic, or rather paramorphic, 

 after hornblende of that type. This hornblende is, therefore, 

 in all probability the oldest constituent of the rock, with the 

 exception of such minerals as sphene and apatite. The conversion 

 of the green hornblende into augite appears to indicate some 

 change of conditions, probably an increase of temperature at a 

 pressure less than that under which the hornblende was originally 

 formed. After the formation of the augite the next mineral to 

 crystallize must have been the brown hornblende, and probably at 

 a still later date the small patches of segirine-augite already 

 referred to were deposited on the augite. 



When the separation of the coloured minerals (with the possible 

 exception of the aegirine-augite) was completed, a certain amount 

 of nepheline appears to have crystallized out in more or less idio- 

 morphic crystals (see p. 44, text & first footnote), leaving a magma 

 with approximately the composition of analcime. This mineral 

 then began to crystallize out from different centres. As the 

 crystals grew, each pushed back the constituents which were already 

 solidified, thus clearing the coloured and other previously. formed 

 minerals away from an approximately spherical area that was 

 gradually increasing in size (see pp. 39 & 41). From time to time, 

 new centres of crystal-growth appeared whence the same process 

 was repeated. This gradual outward growth of the analcime- 

 crystals continued, till the minerals that made way before them 

 came into contact one with the other, and there was no more room 

 for symmetrical expansion ; but the magma which remained in 

 the interstices continued to solidify outward from the analcimes 

 already formed, in crystalline continuity with them, until all 

 became solid. The result is the apparent anomaly that, although 

 the crystallization of the analcime has formed and shaped the 

 spherical spaces that are now so marked a characteristic of the rock, 

 yet the crystals occupying these spaces have no boundary except the 

 surface of the other usually idiomorphic minerals with which 

 they come" into contact, or of adjoining crystals of analcime. The 

 last-mentioned boundary is not visible even in polarized light, on 

 account of the isotropic character of the analcime, so that the whole 

 matrix appears to be continuous ; but the varying directions of the 

 inclusions indicate that it is really made up of many diff'erently 

 orientated crystals — with spherical nuclei of varying size and rami- 

 fications beyond, between the earlier minerals.^ 



^ It is of some interest to compare these spheres with the similarly shaped 

 spaces filled with ' interstitial matter ' described by Mr, Teall, and shown 

 by him to be formed by the infilling of steam-cavities with the residual magma 

 of the rock [32], 



