156 PROF, T. T. GROOM ON THE IGNEOUS ROCKS [Feb. I9OI, 



10. On the Igneous Rocks associated with the Cambrian Beds of the 

 Malvern Hills. By Prof. Theodore T. Groom, D.Sc, M.A., 

 F.G.S. (Read December 19th, 1900.) 



[Plate VII.] 



Contents. 



Page 



I. Historical Summary 156 



II. IntrodiU'tion 158 



III. Ampbibole-bearing Rocks of Andesitic Habit 158 



IV. Aiigite-Basalt 167 



V. Olivine-Basaits 168 



VI. Olivine-Diabases . 172 



VII. Mutual Relations between the Rocks described 175 



VIII. Comparison of the Malvern Intrusives with the Igaeous 



Rocks of other Districts 177 



IX. Supposed Pjroclastic Rocks 179 



X. The Intrusive Character of the Rocks ISO 



XI. Diite of Intrusion 181 



XII. Relation of the Igneous Rocks described to those of the 



Archaean Massif 182 



XTII, Conclusions 182 



I. Historical Summary. 



The crystalline core of the Malvern Hills was at first supposed by 

 Horner^ to be composed of igneous rocks intruded into the adjacent 

 strata. Murchison at a later date imagined that the HoUybush 

 Conglomerate and Hollybush Sandstone were submarine volcanic 

 grits discharged from a fissure along which later the igneous 

 mass of the Malverns burst through.^ Phillips, however, adduced 

 strong reasons for believing that the crystalline rocks had not been 

 intruded into the Palaeozoic strata ^ ; he was, moreover, the first to 

 recognize igneous rocks in the lowest portions of the latter. He 

 described ' felspathic dykes ' and * interposed masses ' in the 

 Hollybush Sandstone ; and ' porphyritic and greenstone-masses, 

 which, erupted from below, have flowed in limited streams over 

 the surface of the Black Shales.'^ He pointed out that neither dykes 

 nor bosses of trap occur in any of the strata above the Black (and 

 Grey) Shales ; though some of them, including those near Bronsil 

 (numbered 108, 247, 248, & 249 in the present writer's map^), 

 are situated in, or on, the upper part of the Shales. In the vertical 

 section (p. 51) in Phillips's work, and in the horizontal section 

 (No. 13) accompanying the memoir,® they are placed between 



1 Trans. Geol. Soc. ser. 1, vol. i (1811) p. 281. 



2 ' Silurian System ' 1839, pp. 416, 418. 



3 Phil. Mag. vol. xxi (1842) p. 288, and Mem. Geol. Surv. Gt. Brit. vol. ii, 

 pt. i (1848) pp. QQetseqq. 



4 Mem. Geol. Surv. Gt. Brit. vol. ii, pt. i (1848) pp. 52, 53, & 56. 

 ^ Quart. Journ. Geol. Soc. vol. Iv (1899) pi. xiii. 



^ Reproduced in Sir A. Geikie's * Ancient Volcanoes of Great Britain ' vol. i 

 (1897) p. 170. 



