Vol. 67.] IN THE OLD RED SANDSTONE OF MONMOUTHSHIRE. 469 



by the rapid cooling of the magma, separated by the explosive 

 violence of the intrusion into small fragments, mixed confusedly 

 with cold chips of the adjacent sandstone. 1 It is clear that the 

 fragmental character of the rock, both igneous and enclosed sedi- 

 mentary, testifies to the violence of the intrusion ; but an added 

 interest is the significant difference between this glassy variety of 

 the magma and the usual (analcite) matrix, already described. In 

 other parts of the rock, small areas of glass with a very dark-brown 

 colour by transmitted light, can be made out. But, in every case 

 noted, the glassy patch surrounds or partly fringes a xenolith, 

 and is sharply separated off from the normal analcite-matrix 

 {PI. XXXVII, fig. 5) ; or, if the glass occurs in somewhat larger 

 patches, it is invariably filled with innumerable small xenoliths. 

 The fact that we have, side by side as it were, the brown and yellow 

 basic glass and the colourless isotropic substance which ordinarily 

 plays the part of ground-mass in this rock, affords additional 

 evidence that this colourless substance is not glass, but analcite. 2 



As to whether the analcite of the ground-mass is primary, that 

 is, pyrogenetic, it is to be observed that, unlike some other rocks of 

 this class containing analcite, the constituents of this Monmouth- 

 shire monchiquite are not all of them fresh ; the olivine in particular 

 is decomposed, and there is much secondary carbonate in the rock 

 (see Analysis I, p. 470). On the other hand, the boundary between 

 the analcite and the locally-developed glass described above is 

 invariably a sharp one, not in the least suggestive of the conversion 

 of glass into analcite; and, moreover, the mineral of the ground- 

 mass is interstitial and without crystal- outlines, unlike'the secondary 

 analcite of the ocelli. Having regard to these points and to the 

 analogy of other analcite r bearing rocks where the mineral inlays 

 the part of ground-mass as in this rock, and where the other 

 minerals are quite fresh, it seems highly probable that the analcite 

 here is primary. 



The magma was apparently very hydrous, and crystallized under 

 pressure and somewhat rapidly, after being violently intruded 

 in a dyke-like or plug-like form into the comparatively cold sur- 

 rounding rocks. Analcite was formed as a sparse ground-mass 

 between the already consolidated minerals, showing 1 here and 

 there in more conspicuous patches, comparatively free from ferro- 

 magnesian minerals and iron-ore ('analcite-phenocrysts' of Pirsson). 

 But, where locally and accidentally there was sudden relief of pressure 

 and shattering of the magma, accompanied by rapid chilling, a8 

 near the small xenoliths of sandstone and marl, the characteristic 

 basic glass of the limburgites, full of minute vesicles, resulted 

 instead of analcite. 



1 The possibility of the local absorption by the magma of silica, etc., from 

 the xenoliths must be taken into account ; but it seems more probable that the 

 operative cause in the formation of the glass was the one stated. 



2 The finely powdered rock, when digested with very dilute (5 per cent.) 

 hydrochloric acid, yields gelatinous silica. 





