566 ME. H. WOODS ON THE IGNEOUS BOCKS [Nov. 1 894, 



37. The Igneous Bocks of the Neighbouehood of Buii/th. By 



Henet Woods, Esq., M.A., F.G.S. (Bead June 20th, 



1894.) 



Contents. 



Page 



I. Introduction and Bibliography 566 



II. TheDiabase-Porphvrite 567 



III. TheAndesites 569 



IV. The Andesitic Ashes 571 



V. TbeEbyolites 573 



VI. The Diabases 574 



VII. Conclusions 577 



Geological Map of the District 568 



I. Introduction and Bibliogeaphy. 



In south-west Radnorshire a series of igneous rocks associated 

 with beds of Ordovician age stretches from uear Builth in the south 

 to beyond Llandrindod in the north. This area is surrounded on 

 all sides, except the north-west, by Silurian rocks, and has a length 

 of about 7g miles, and a width varying from 2 to 5 miles. The 

 best account of the geology of this district is that given by Murchison 

 in the ' Silurian System.' Short descriptions or notes have also 

 appeared in memoirs and papers by De la Beche, Phillips, M'Coy, 

 llamsay, Symonds, Lapworth, and Butley. The district is shown 

 on sheets 56 S.W. and 56 S.E. of the Geological Survey, and was 

 mapped by the late Sir Andrew Bamsay and by Mr. W. T. Aveline ; 

 the maps were issued in 1850, and were explained by two horizontal 

 sections on Sheets 5 and 6, but no descriptive memoirs were 

 published. 



The following is the literature relating to this area of Ordovician 

 and associated igneous rocks : — 



Murchison, E. I.— 'The Silurian System' (1839), pp. 314, 324-335, pi. xxxiii. 



figs. 5-7. 

 Murchison, E. I.—' Siluria,' 5th ed. (1872) pp. 59, 81. 

 De la Beciie, H. T. — ' On the Formation of the Eocks of South Wales and 



South-western England,' Mem. Geol. Surv. vol. i. (1846) p. 22. 

 Phillips, John. — ' The Malvern Hills,' etc., Mem. Geol. Surv. vol. ii. pt. i. 



(1848) pp. 227-315, 327. 

 M'Coy, P.—' British Paleozoic Fossils ' (1852), pp. 354, 369, 373-74. [Lists of 



fossils.] 

 Eamsay, A. C, H. W. Bristow, H. Bauerman, and A. Geikie. — ' Catalogue of 



the Eock Specimens in the Museum of Practical Geology,' 2nd ed. (I860) 



pp. 112, 197, 211, 212, 248. 

 Symonds, W. S.— 'Eecords of the Eocks,' 1872, p. 93. 

 [Newton, E. T.] — 'A Catalogue of the Cambrian and Silurian Fossils in the 



Museum of Practical Geology,' 1878, pp. 23-29. [Lists of fossils.] 

 Lapworth, C- Ann. & Mag. Nat. Hist. ser. 5, vol. iv. (1879) p. 339. 

 Butley, F. — ' The Felsitic Lavas of England and Wales,' Mem. Geol S urv 



1885, p. 20. 



In the present paper I propose to deal with the igneous rocks of 

 the southern half of this district — that lying between the town of 



