182 PKOF. T. T. GROOM ON THE IGNEOUS ROCKS [Feb. I9OI, 



XII. Eelation of the Igneofs Eocks described to 



THOSE OP THE ArCH^AN MaSSIE. 



Seeing that the intrusive rocks which have invaded the Cambrian 

 strata must have passed through the underlying Archaean, it might 

 be expected that representatives of them would be found in the 

 neighbouring Archaean complex, and that the latter should, there- 

 fore, contain not only pre-Cambrian but also later igneous masses. 

 The circumstance that the three chief types of rock — namely, 

 diorite, granite, and felsite, which according to Dr. Callaway^ 

 enter into the composition ol the Malvernian complex, have under- 

 gone an apparently pre-Cambrian foliation, militates against the 

 idea that either the granite or the coarser diorite can be the 

 plutonic representative of any rock seen in the Cambrian strata. 

 It is, however, conceivable that certain of the rocks regarded as 

 diorite may belong to the same series as the hornblende-rocks in 

 the Cambrian Beds ; but, until the diorites of the Malvern chain are 

 better known, it will be impossible to deal with this question. 

 Augite-bearing rocks appear to be rare in the Malvernian. Pro- 

 terobases and epidiorites are described by Mr. Teall,^ but rocks of 

 this description do not invade the Cambrian. The same writer, 

 however, detected among the rocks on Swinyard Hill an ophitic 

 diabase precisely similar to those seen in the Cambrian.^ 



Augite-bearing rocks occur in the Warren House (Uriconian) 

 Series,^ but from Mr. Eutley's descriptions and from a personal 

 examination of Mr. H. D. Acland's slides by the present writer, it 

 appears that these rocks are of a type different from any seen in the 

 Cambrian. 



We have, then, no certain proof that, with the exception of the 

 olivine-diabase, any of the rocks described iu the foregoing pages 

 have invaded the Archaean. Prof. Watts informs me that this is also 

 the case with the allied intrusive rocks of other parts of the English 

 Midlands. 



XIII. Conclusions. 



The Cambrian of the Southern Malverns is associated with a series 

 of igneous rocks which have commonly been regarded as volcanic. 

 There are, however, among them no truly vesicular rocks and 

 no tuffs such as have been mistakenly described, and the rocks 

 are probably all intrusive. 



Petrographically the assemblage may be divided into three, 

 structurally and mineralogically very constant, types, and a fourth 



^ Quart. Journ. Geol. Soc. vol. xliii (1887) pp. 526 & 527. 



2 . British Petrography ' 1888, p. 245. 



^ Loc. cit. The rock (1140) to which reference is made may be seen in the 

 Natural History Museum : it contains abundant serpentinized olivine. 



4 F. Eutley, Quart. Joi^n. Geol. Soc. vol. xliii (1887) pp. 497-99, and H. D. 

 Aclaud, ibid. vol. liv (1898) pp. 659 & 561. 



