180 PBOP. T. T. GROOM ON THE IGNEOUS KOCKS [Feb. IQOi, 



remarks that ' one of these may be observed on the east side of 

 the (]uarry at the south-west base of Midsummer Hill ' ; he states, 

 however, that the ash ' is, in part, of the striioture of sandstone, and 

 in part felspalhic, with traces of epidote.' The analyses of this rock 

 (XXIII & XXIV) show it to be of a decidedly more acid character 

 than any of the igneous rocks associated with the Sandstone or with 

 any of the Malvern Cambrians. I have nowhere been able to recognize 

 in the Sandstone beds which resemble tuffs, and have examined what 

 I believe to be the particular layer spoken of by Timins. It occurs 

 along one of the joint-planes which ran parallel to the surface of a 

 dyke (M 112), and crosses the bedding, which is here very obscure. 

 The supposed ashy layer appears to be nothing but a portion of the 

 Sandstone, modified along a line of crushing and displacement. 



After careful search in the Black and Grey Shales I have been 

 equally unable to detect any traces of pyroclastic rocks. Crush- 

 breccias are occasionally seen on a very small scale, and some of the 

 rocks have somewhat the appearance of sandstones and fine tuffs. 

 Indeed, Phillips speaks of sandstones interbedded with the shales,^ 

 and HoU describes ashes and grits in the ' Coal-Hill Band.' ^ 

 Symonds also mentions volcanic grits in the Black Shales.^ Micro- 

 Bcopic examination shows that the supposed sandstones, grits, and 

 ashes are nothing but weathered diabases and basalts (see pp. 169, 

 174).^ 



It must be concluded, then, that pyroclastic rocks are 

 probably altogether absent from the Cambrian Beds 

 of the Malvern area. 



I 



X. The Intrusive Character of the Bocks. 



The camptonitic dykes and bosses in the Hollybush Sandstone 

 must evidently be removed from the category of lavas, and so must 

 the bosses in the Black and Grey Shales. There remain only the 

 sheets and supposed laccolites intercalated in the Shales, and more 

 rarely in the Sandstone. Of these the diabases, as might be expected 

 from their general occurrence as dyke-rocks, are at least in part intra- 

 6ive(p. 173). In the case of the basalts the question is more difficult; 

 the materials are such as might belong either to a lava or to a small 

 sill: the absence of glass, and the rarity and small size of the 

 steam-vesicles rather support the idea of an intrusive origin. The 

 basalts in many cases do not appear to have greatly affected the 

 Shales, as was observed by Symonds (he. cit,) ; but in other cases the 

 Black Shales have been bleached in a somewhat sporadic manner, and 



1 Mem. Geol. Surv. Gt Brit. vol. ii, pt. i (1848) p. 54. 



3 Quart. Joarn. Geol. Soc. vol. xxi (1865) pp. 89 & 90. 



3 « Records of the Rocks ' 1872, p. 73. 



* It may be pointed out here that, although the grits described by previous 

 observ^ers appear to be non-existent, a series wliich could hardly have baen seen 

 by them has been detected by the writer, but these are in no way pyroclastic: 

 see Rep. Brit. Assoc. 1900 (Bradford). 



