Vol. 67.] IN THE OLD RED SANDSTONE OF 3IONMOUTHSniRE. 473 



entirely surrounding the olivine, bear an unmistakable resem- 

 blance, both in colour and association, to the chrome-diopsido 

 of some of the picrites and lherzolites. In places, an approach to 

 the diallagic structure was observed. 



Irregular grains and patches of translucent chromite or pico- 

 tite x are plentiful, up to 6 mm. across, usually of a deep 3'ellowish 

 colour, but sometimes yellow-brown, and in one or two small 

 patches they have a deep green colour, possibly indicating pleonaste. 

 A bright yellow-green substance often extends for some distance 

 from the margin of the chromite-grains, and is found also in the 

 cracks of adjacent minerals ; it is, presumably, chrome-oxide. 



The rock is unquestionably a peridotite, and approaches to a 

 coarsely-crystalline picrite (PL XXXVII, fig. 6). The question 

 arises as to how far these corroded xenoliths of picrite are related 

 to the monchiquite. The diopside, olivine, and chromite of the 

 picrite are minerals which occur as phenocrysts in the monchiquite, 

 except that the augite of the picrite has the additional diallagic 

 structure slightly developed, while the olivine tends to pass into the 

 iddingsite product, rather than into the usual serpentine. More- 

 over, the chemical composition of the picrites agrees very closely 

 with that of our monchiquite. For purposes of comparison, the 

 analysis of a picrite is placed alongside that of the Monmouth- 

 shire monchiquite, and the close similarity is at once detected : — 



I. II. 



Per cent. Per cent. 



Si0 2 40-26 42-85 



TiO, 2-26 



FeO" 714 6-86 



Fe 2 3 2-86 6-27 



A1 2 3 1022 10-42 



Cr 2 3 trace 



MnO 0-20 



CaO 13-43 11-84 



MgO 8-75 901 



K„0 1-32 1-61 



Ka 2 1-51 1-65 



P.,0- 0-65 



S0 3 0-62 



CO„ 5-80 5-88 



H 2 Oatl05°C 1-24 j Q ^ 



H 2 above 105° C 353 J * ' U 



Totals 99-79 99D9 



I = Monchiquite near ' Great House,' Golden Hill, Monmouthshire. (Anal. 

 S. J. Johnstone.) 

 II = Altered picrite from Sohle (see J. J. H. Teall, ' British Petrography 

 1888, p. 130). This has suffered a change whereby magnesia has been 

 removed and lime introduced. A similar change in composition has 

 been effected in the Monmouthshire monchiquite and its picritic 

 inclusions. 



1 The heavy particles were separated from the powdered rock, and gave the 

 chromium-green colour with the borax bead. 



