Vol. 67.] IN THE OLD RED SANDSTONE OF MONMOUTHSHIRE. 471 



a low percentage of silica l and alumina, and a high percentage of 

 lime, magnesia, and iron, while the alkalies are only moderate in 

 amount. Thus the composition is that of a typical hasic lam- 

 prophyre, which is fully borne out by the petrographical characters 

 already described. 



The presence of phenocrysts of augite, biotite, and olivine, the 

 abundance of augite-prisms and biotite-flakes in the matrix, the 

 absence of felspars, and the presence of analcite as a colourless 

 isotropic base, mark it out as a mouchiquite. 2 



That the gap between the monchiquite and the limburgite types 

 is not considerable would appear probable, seeing that both occur 

 in close juxtaposition as variations of the same rock ; not, however, 

 by a change of the basic glass into analcite, but by the practically 

 simultaneous formation of glass and analcite under somewhat 

 different conditions, apparently in the main a different rate of 

 cooling. 



An unusual amount of carbon dioxide appears in the analysis, 

 and this was to be expected in view of the great development of 

 carbonate revealed by the microscope. Some of this carbonate may 

 possibly be in the form of dolomite, but most of it is calcite. The 

 finely-powdered rock, and the ocelli in hand-specimens, effervesce 

 vigorously with very dilute hydrochloric acid. Thus, in comparing 

 the composition of this rock with that of others showing less 

 carbon dioxide, it will be necessary to make some deduction from 

 the lime-content. 3 The soda is rather smaller in amount than 

 anticipated, and less than in most rocks that have been described 

 as monchiquites. 4 The alkalies of the monchiquite group vary 

 greatly in amount ; in nearly all cases the soda is in excess of the 

 potash. 



Y. The Oiivine-Augite-Eock Inclusions. 



We will now consider the inclusions or ' nodules ' of somewhat 

 coarsely-crystalline rock, which contain the conspicuous, bronzy, 

 mica-like substance, and are obviously quite distinct from the pheno- 

 crysts of augite and biotite, as well as from the xenoliths of Old 

 Eed Sandstone. 



Seen under the microscope, the junction of one of these inclusions 

 and the containing rock shows clearly a corrosion-border of faintly- 

 polarizing granular material with much iron-oxide ; and the 



1 Three additional determinations of silica were made of three different 

 specimens of the rock in the laboratory of University College, Cardiff. The 

 results -were 4075, 41 "04, and 42 - 53 per cent, respectively. 



2 The term monchiquite as here used is in the sense advocated by 

 L. Y. Pirsson, who showed that the ' pitchstone-glass ' of the original mon- 

 chiquite of Hunter & Eosenbusch is really analcite (see Journ. Geol. Chicago, 

 vol. iv, 1896, p. 679). 



3 If all the carbon dioxide is combined with lime, and all the calcium 

 ■carbonate is of extraneous origin, the percentage of lime in the analysis would 

 be reduced to the extent of about 7'4. 



4 The monchiquite from the Castle - Mountain district, Montana (see 

 Analysis Y, p. 470), contains only T21 per cent, of soda. 





