460 PEOF. W. S. BOTJLTON ON A MONCHIQUITE INTKT7SION [ISToV. I9II, 



16. On a Monchiquite Intrusion in the Old Red Sandstone of 

 Monmouthshire. By Prof. William S. Boulton, B.Sc, 

 Assoc. R.C.S., F.G.S. (Eead June 14th, 1911.) 



[Plate XXXVII — Microscope-Sections.] 



Contents. 



Page 



I, Position and Field-Kelations s 4BO 



II. Field-Characters and Kelations to the Contact-RocKs 461 



III. Petrographical Details 464 



IV. Chemical Composition 470 



V. The Olivine-Augite-Pock Inclusions 471 



VI. Affinities with other Bocks 474 



VII. Summary 475 



I. Position and Field-Relations. 



The igneous intrusion here described, and hitherto unrecorded, is 

 situated in Monmouthshire in lat. 51° 40' H"., long. 2° 49' 30" W. 

 Following the main road from Chepstow to Usk for about 7^ miles, 

 and then branching off southwards for another mile and a half, we 

 come to a place which is marked on the 6-inch Ordnance map 

 of the district as the ' Great House ' ; and about 250 yards north 

 of it, the rocks in question are exposed. Some 70 yards north- 

 west of the carriage-drive leading to the house (where I first 

 noticed the rock which was being used to 'metal' the drive), a small 

 quarry has been excavated in the rising ground. It is in this 

 quarry that the main exposure of the monchiquite is seen, and 

 from which most of the material described in this paper was 

 obtained. (Fig. 1, p. 461.) 



The quarry measures about 20 by 25 yards and is some 20 feet 

 deep. A few yards west of it is a small, shallow opening, from 

 which the igneous rock has evidently been taken, and some 

 of it may still be seen in situ cropping out through the grass. 

 Still farther west, over the fence, a similar hollow is seen, and a 

 little north of it is an old quarry measuring 20 yards across and 

 of considerable depth, with a pool at the bottom and the sides 

 overgrown with bushes and long grass. In this old disused quarry 

 no rock is visible in position ; but large, loose lumps of the 

 monchiquite and slabs of sandstone are lying about. 



The monchiquite is intrusive in the Old Red Sandstone, which 

 covers so large an area in this part of Monmouthshire, though the 

 exact stratigraphical position is uncertain. From general traverses 

 through the district, it appears to lie near the base of the upper 

 division of the system or Brownstones, made up of red sandstones 

 and conglomerates, with subordinate bands of red marl. 



