178 PBOP. T. T. GROOM ON THE IGNEOFS ROCKS [Feb. IQOI, 



The amphibole-bearing rocks of the Malvern Hills do not resemble 

 tbe ordinary hornblende-basalts, andesites, or porpbyrites. They 

 appear to find their closest analogues in the Warwickshire 'diorites,' 

 originriUy described by Samuel AUport/ The name camptonite 

 has been suggested by Prof. Watts for these Warwickshire rocks." 

 He has kindly examined some of my slides, and considers that 

 they conform to the Nuneaton type. Brief descriptions of some 

 of the Warwickshire rocivs have been published by Messrs. Teall,^ 

 Waller,'* Rutley,^ and Prof. Watts {loc. cit.). The last-mentioned 

 has kindly sent me a number of slides belonging to Prof. Lapworth 

 and himself; and, thanks to the courtesy of Mr. Teall and Mr. Prior, 

 I have been able to examine many other slides of these rocks. 

 One of the commonest types in the Warwickshire district has been 

 figured by Mr. Teall.® This consists of idiomorphic needles of 

 brown hornblende, with iron-ores and apatite, set in a holo- 

 crystalline coarse- or fine-grained matrix of more or less lath-shaped 

 felspars. This type of rock perhaps corresponds most nearly 

 with the Malvern amphibole-bearing series, but phenocrysts of 

 augite are absent. Equally common is a type which differs from 

 the foregoing in the presence of phenocrysts of olivine, which 

 are often grouped together. These rocks seem to correspond with 

 the Malvern olivine-basalts, but they possess amphibole instead 

 of augite. One rock (554) from Dosthill, in the AUport Collection 

 in the Natural History Aluseum, seems to connect the two Malvern 

 porphyritic types : in a groundmass which much resembles that of 

 these rocks, and of the augite-basalt (M 101), phenocrysts of olivine 

 and augite are seen, but no amphibole.'^ 



Taking the rocks as a whole, in spite of differences of development, 

 there seems to be a considerable analogy between the rocks of 

 the Malvern and Nuneaton areas. In both cases we have a series 

 of small intrusive masses chiefly in the form of sills and dykes of 

 more or less basic material which have invaded the Cambrian, but 

 no later formations. Amphibole, augite, and olivine are charac- 

 teristic minerals, and the amphibole was probably of the same 

 peculiar character, and has undergone similar modifications in each 

 case. There is, moreover, a marked tendency in both cases for the 

 ferromagnesian minerals to give rise to serpentine rather than 

 chlorite. In each district camptonitic features are developed in 

 some of the rocks. This is seen in the complete absence of quartz, 

 the rarity of phenocrysts of felspar, and the prevalence of the ferro- 

 magnesian minerals ; among which biotite is absent in each case. 

 Moreover, ophitic rocks, such as commonly are associated with camp- 

 tonites, and which indeed are sometimes classed with them, are 

 present in both areas. 



^ Quart. Journ. Geol. Soc. vol. xxxv (1879) p. 637. 



2 Proc. Geol. Assoc, vol. xv a898) p. 394. 



3 ' British Petrography ' 1888, pp. 250-51. 



* Geol. Mag. 1886, p. 322. ^ jhid. pp. 557 et seqq. 



« • British Petrography ' 1888, pi. xxix, fig. 2. 



'' It is interesting to note that or' all the localities in which Cambrian rojka 

 are exposed in Warwickshire, Dosthill is the nearest to the Malverua, 



