Page:VCH Herefordshire 1.djvu/71

 GEOLOGY odorulite. The amount of lime present in these ' Upper Cornstones,' as Symonds called them, is often considerable, and water that has percolated through them on issuing forth frequently forms a deposit of travertine or calcareous tufa. Symonds records the occurrence of such a deposit near Chapel-le-fin. Upper Division. — Two subdivisions of this series can be recognized (i) an upper, consisting of yellow, grey, and red sandstones with occasional bands of red shale, and (2) a lower, in which red marly beds, quartz-grits, and conglomerates,'^^ predominate. The beds which cap the edge of the Black Mountains and extend south-westwards for some distance, forming the surface of the uplands, are the grits and conglomerates. As might be expected the soil to which they give rise is very light, but nevertheless well suited for sheep-farming. The higher beds come on farther back from the edge of the Black Mountains, and dip under the Carboniferous rocks of the great South Wales Coalfield. For reasons which have been already outlined, beds belonging to this Upper Division are absent from the neighbourhood of Ganarew ; but they appear again in the hills of the Forest of Dean to the south of Ross, where the highest strata are exposed at several localities. The lane which ascends the steep north-western slopes of the Great Doward, near Whitchurch, exposes the Conglomerate Beds, and pebbles of quartz and other rocks strew the surface of the ground. Higher up are poor exposures of the topmost strata, somewhat yellowish in colour. Owing to the dip, these beds are seen at the foot of the cliffs on the opposite side of the Wye below Symond's Yat. Similar beds have been observed at Howie Hill ; but by far the best section, which has claimed the attention of De la Beche, Symonds, J. Jones and W. C. Lucy,'^^ and Mr. E. B. Wethered," is that in the deep cutting near the ' Hawthorns ' on the road from Ross to Drybrook. Here the complete transition from beds which are unequivocally Devonian, into those which are with equal certainty Carboniferous, is admirably displayed. T^he Bartestree Igneous Rock. — About three miles to the east of Hereford is a very interesting intrusion of igneous rock. It is the only igneous rock that pierces the Old Red Sandstone of Herefordshire, and on account of its hardness has been much quarried in the past. Now, however, the working is abandoned, and the quarry remaias as a deep cutting marking out very clearly the direction of the intrusive mass, namely north-east and south-west. Many writers have referred to the rock as a greenstone,^^ and La Touche''^ describes it as a diorite, but Professor S. H. Reynolds informs me that it is a dolerite. The main mass is a dark, compact, rather fine-grained rock, in which crystals of augite can be detected with the naked eye. Under the microscope a slice of the rock exhibited phenocrysts of plagioclase, augite, and magnetite, in a fine-grained ground-mass, but there was no olivine. In " See H. C. Moore, Trans. Woolhope Nat. F.C. 1900, to April, 1902 (1903), pp. 227-8. ™ Proc. Cotteszvold Nat. F.C. iv (1866-8), pp. 175-93. " Quart. Journ. Geol. Soc. xxxix (1883), pp. 21 1-16. " Phillips, Mem. Geol. Surv. ii, pt. i (184.8), p. 180 ; H. E. Strickland, Quart. Journ. Geol. Soc. viii (1852), p. 384. '' Quoted by G. H. Piper in Trans. Woolhope Nat. F.C. 1890-2, p. 166. 23