Vol. 57.] IGNEOUS ROCKS AMONG THE MAL7EEN CAMBRIANS. 1T7 



gravity ; also in tteir structure, mineralogical ccmpositicn, distri- 

 bution, ard to a certain extent iu their mode of occurrence. 

 While the olivine-bcaring rocks are chiefly characteristic 

 of the Upper Cambrian Shales, in which they take the form 

 of sills and small laccolites, the amphibole-bearing rocks 

 also invade extensively the Hollybush Sandstone, chiefly 

 in the form of dykes and bosses; and even where the Upper Cambrian 

 Shales are invaded the form assumed is oiten that of a boss. The 

 groundmass is usually much finer grained than in the olivine- 

 basalts, and flow-structure is invariably seen ; the phenocrysts in 

 the two series differ entirely, and secondary biotite, abundant in 

 the basalts, is rare in the amphibole-bearing rocks. A fourth 

 type of rock (augite-basalt) presents resemblances on the one 

 hand to the 'andesitic camptonites,' and on the other to the olivine- 

 basalts (see p. 167) ; but it is not intermediate between these types, 

 for it contains neither amphibole nor olivine. 



VIII. Comparison of the Malvern Intrusives with the 

 Igneous Eocks of other Districts. 



The Malvern diabases described in the foregoing pages in some 

 respects resemble the oli vine-diabases and dolerites which have 

 invaded the Old Eed Sandstone and Carboniferous rocks of Here- 

 fordshire, Worcestershire, the Clee Hills, the Forest of Wyre, and 

 the Central English Midlands. Mr. Teall, however, points out 

 that as a whole the olivine in these rocks is rich in iron, and that 

 the augite is deeply coloured.^ The Malvern diabases do not 

 share these features. They differ from the Ordovician diabases 

 of the Lake District, North Wales, and Shropshire in the presence 

 of olivine, and in other respects.^ They differ also from the 

 Warwickshire ophitic ' diorites ' in containing augite in place of 

 hornblende. 



Similar statements may be made with reference to the olivine- 

 basalts. The Malvern examples belong to a somewhat unusual 

 type, and differ from the usual Ordovician basalts of the Lake District 

 and North Whales in the presence of olivine (Teall and Harker, 

 he. cit.). Olivine-basalts of Cambrian or slightly earlier age, and 

 again of a different character, have been described by Sir Archibald 

 Geikie from South W^ales.^ Olivine-basalts in the form of flows 

 and dykes, more nearly allied to the Malvern basalts, abound in 

 Scotland and Ireland, and occur to a more limited extent in the 

 English Midlands * ; but these newer rocks, where porphyritie, 

 appear to diff'er for the most part in the greater variety of the 

 phenocrysts. 



1 ' British Petrography ' 1888, p. 214. 



^ Ihicl. pp. 214 et seqq. ; Harker, ' Petrology for Students ' 1895, pp. 113 

 etseqq.; Watts, Proc. Geol. Assoc, vol. xiii (1894) p. 338. 



^ Quart. Journ. Geol Soc. vol. xxxis (1883) p. 304; see also Harker, 

 op. cit. p. 175. 



* Teall, • British Petrography' 1888, pp. 186 et seqq. ; and Harker, 'Petro- 

 logy for Students ' 1895, pp. 177-79. 



a J. G. S. No. 225. N 



