|_ 203 



XVJI. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY, 



[Continued from Vol. xxii. p. 820.] 



June 14th, 1911.— Prof. W. W. Watts, Sc.D., M.Sc, F.R.S., 

 President, in the Chair. 



r |\HE following communications were read; — 



1. ' On a Monchiquite Intrusion in the Old Red Sandstone of 

 Monmouthshire.' By Prof. William S. Boulton, B.Sc, Assoc. R.S.C., 

 F.G.S. 



The paper describes a hitherto unrecorded monchiquite, intruded 

 into the Upper Old Red formation of Monmouthshire, about midway 

 between Chepstow and Usk. The precise manner of its intrusion is 

 doubtful ; reasons are given for regarding it either as a dyke with 

 a north-westerly trend or as a volcanic plug (bysmalith). The dis- 

 turbance and metamorphism of the contact-rocks (seen only at its 

 eastern edge) are dealt with, as also the rounded lumps of marl and 

 subangular blocks and chips of sandstone incorporated in the igneous 

 rock. 



The monchiquite, which is described in detail, contains unusually 

 large phenocrysts (measuring up to 5 and 6 inches) of augite 

 (chrome-diopside) and biotite, generally much corroded. Rounded 

 lumps or ' nodules ' of olivine-augite rock with chromite are also 

 included, the olivine now being represented by secondary products, 

 including- iddingsite. A second generation of purple idiomorphie 

 augite, biotite, and decomposed olivine occurs porphyritically in the 

 ground-mass, with occasional granules of quartz and chromite. 



The ground-mass is a felt of minute elongated augite-prisms, 

 magnetite-grains, and flakes of biotite, and the remaining space is 

 occupied by analcite enclosing apatite-needles. Reasons are adduced 

 for regarding the analcite of the ground-mass as primary. Ocelli 

 filled with secondary carbonate, chlorite, analcite, etc. are common. 



A complete analysis of the rock is given, which bears out the 

 petrograpbical evidence that it is a very basic lamprophyre belonging 

 to the monchiquite group. It is compared with other rocks of the 

 group, and in particular with the monchiquites and camptonites of 

 Colonsay, with which it has many points in common. Finally, its 

 age and possible connexion with the only other known intrusion 

 into the Old Red Sandstone of the South Wales area — that of 

 Bartestree, near Hereford — are referred to. 



2. ' Notes on the Culm of South Devon : Part I.— Exeter District.' 

 By Frederick George Collins, F.G.S. ; with a Report on the Plant- 

 Remains by E. A. Newell Arber, M.A., F.G.S., and /Notes on the 

 Cephalopoda, by George C. Crick, Assoc.R.S.M.. F.G.S. 



