Vol. 67.] IN" THE OLD RED SANDSTONE OF MONMOUTHSHIRE. 475 



has been published, bub it is significant that one of the variants of 

 that composite dyke is a dark, heavy, aud very basic analcite- 

 dolerite or teschenite, with a silica-percentage of 43*03 and 

 specific gravity = 2-88 (op. tit. p. 509). 



With regard to the age of the Monmouthshire intrusion, 

 the only certain fact is that it is later than the Upper Old Red 

 Sandstone. In the memoir referred to above, Dr. Flett discusses the 

 age of the monchiquite and olivine -dolerite dykes of Colonsay that 

 have a north-westerly trend, and says that 



' they belong to a series which has a wide distribution in Argyllshire and the 

 adjacent part of the West Highlands.' 1 



That they are post-Carboniferous seems certain, but whether they 

 are of the same age as the Tertiary dolerite-dykes of Skye and other 

 parts of Western Scotland is still an open question, and need not 

 be discussed here. 



The Beinn Dearg dykes of the Red Hills of Skye, which are low 

 in silica, comparatively poor in alumina, and rich in magnesia, are 

 regarded by Mr. Harker as a 



' highly specialized derivative from the hypothetical common stock.' 2 

 It may be that the monchiquites of Colonsay are a still more pro- 

 nounced variant in this same basic series of Western Scotland. 



In much the same way, the Monmouthshire monchiquite and the 

 dolerite of Bartestree may be related to each other, and derived 

 ultimately from a common magma; so that, whereas the teschenite 

 of the Bartestree dyke shows some considerable departure from 

 the normal stock, the monchiquite may be regarded as a more 

 extreme variant. 



VII. Summary. 



The rock here described is the only example of a monchiquite so 

 far recorded in England and Wales. It is intrusive in the Upper 

 Old Red Sandstone of Monmouthshire, either as a wide irregular 

 dyke or a plug. 



It is remarkable for its large corroded phenocrysts of augite, 

 biotite, and olivine. 



It has incorporated many lumps and chips of the country-rock, 

 with the usual metamorphic changes in the xenoliths ; and in their 

 vicinity the rock becomes locally, but to a very subordinate extent,, 

 a limburgite, with the more normal glassy base. 



Cognate xenoliths of picrite are also included, which probably 

 represent the more abyssal equivalent of the monchiquite. 



Petrographically and chemically it is very like the monchiquites 

 and camptonites of Scotland, especially of Colonsay; and it may be 

 regarded as a specialized and ultrabasic variety of the same magma 

 that yielded the analcite-dolerite of Bartestree, near Hereford. 



1 ' The Geology of Colonsay & Oronsay, with part of the Eoss of Mull ' 

 Mem. Geol. Surv. 1911, p. 41. 



2 ' The Tertiary Igneous Eocks of Skye ' Mem. Geol. Surv. 1904, p. 328 (see 

 analysis on p. 325). 



Q. J. G. S. No. 268. 2 



