'ol. 67.] IX THE OLD RED SANDSTONE OF MONMOUTHSHIRE. 



463 



red, nearly black, inside, and weather into beautifully- regular 



concentric shells. Some have an internal, finely-nodular structure, 



breaking up like a perlitic pitchstone, doubtless due to shrinkage 



after having been heated up in the magma. 



Pig. 2. drawn natural size, 

 2. — Fine-grained Old Red 



Sandstone at the contact with 



the monchiquite, showing par 



Fig. 



(Xatural size.) 



[Dotted areas are dark purple sand- 

 stone. Light areas are deeply 

 stained with yellow liinonite, and 

 the quartz-grains are in part or 

 wholly corroded.] 



shows a somewhat striking 

 appearance of the sandstone 

 on the floor of the quarry, close 

 to the contact. On a fractured 

 or polished surface of the rock, 

 deep yellow areas, not unlike 

 the spherulitic patches of an 

 obsidian or felsite, lie on a dark 

 purplish-red ground. A thin 

 slice further shows that, in the 

 yellow areas, the rock is thickly 

 covered with yellow oxide of 

 iron and the quartz-grains are 

 partly or wholly corroded ; 

 while in the purple areas the 

 sandstone is practically un- 

 altered, except for strings and 



occasional patches of magnetite. 



An examination of this contact, together with the disposition of 

 the included fragments of sandstone at the western end of the 

 quarry, suggests that the magma forced its way upwards, or perhaps 

 obliquely upwards from the north-west, and then ran for some 

 distance eastwards more or less with the bedding of the sandstones. 

 It may then have passed upwards into strata since removed by 

 denudation. 



It will be gathered from the foregoing observations that it is 

 not possible to define positively the boundary-walls of the intrusion, 

 except on this eastern side ; and here the junction is irregular. A 

 reference to the sketch-map (fig. 1, p. 461) shows the small openings 

 immediately to the west of the quarry, in one of which the igneous 

 rock is exposed; and the. old, overgrown quarry, where loose blocks 

 of the monchiquite have been left. 1 The old quarry and plantation, 

 200 yards south-east of the recently worked quarry, shows no trace 

 of igneous rock ; it is only about 8 to 10 feet deep, and was 

 apparently opened up in the sandstones. Thus we may take it as 

 practically certain that the monchiquite crops out for a distance of 

 about 300 feet in a north-north-westerly direction, and for about 

 200 feet in a direction at right angles to that. 



All these openings where the monchiquite is exposed lie on a 

 rounded crest, which, beginning abruptly near the eastern quarry, 

 runs for nearly half a mile in a north-westerly direction across Golden 

 Hill. The trend of the intrusion may be in this north-westerly 



1 Xo record of the working of this quarry could be obtained. 



