924 w. n. hedleston on the chemical composition of 



not true granite, but only an altered sedimentary rock, we have, so 

 far as I know, no cine. 



The sedimentary rocks of the Lizard peninsula are probably 

 about Lower-Devonian age. The great granite masses further north 

 are probably late Carboniferous, or at any rate pre-Triassic. Dole- 

 ritic outbreaks, such as might have been fed by the great gabbro 

 massif, the form of which seems to suggest the probability that it 

 was once deep below a volcanic cone, occur both of Devonian and 

 Carboniferous age ; and the latest known igneous action in " West 

 Wales" is recorded by the Triassic basalts in and about Exeter. 

 We may then venture to conjecture that the intrusion of the lherzo- 

 lite was of later Devonian age, and that both the gabbro and the 

 granite may belong to some part of that immense period when the 

 Coal-measures of Central and Xorthern England were being deposited 

 and afterwards denuded before the " Xew Red " series was formed. 

 Possibly the dark traps may record the slight sporadic igneous action 

 indicated by the Exeter basalts, and so be of Triassic age. The as- 

 pect, however, of some of these seems indeed to suggest that they 

 solidified at no great distance from the surface, and so under condi- 

 tions not very different from those at present existing. Hence it is 

 possible that these veins, together with the phonolite of the Wolf 

 Rock, are remains of a yet later period, the Miocene, during which 

 volcanic action was so rife in Scotland, Central France, and Germany. 



Notes on the Chemical Composition of some of the Rocks of the Lizard 

 District. By W. H. Hedleston, Esq., 1LA., E.G.S. 



These rocks may be roughly divided into four principal groups 

 (without enumerating others of less importance), viz. : — 



1. The Serpentines. 



2. The " Greenstones " in the Serpentine. 



3. The Hornblende Schists. 



4. The Gabbros. 



Features in common. — In all specimens of the above rocks exam- 

 ined by me, alkalies are extremely scarce, if not altogether absent in 

 most cases. They are all basic, rarely containing as much as 50 

 per cent, of silica. 



Differences. — Chemically they divide themselves into two groups: — 

 Group A (poor in lime and alumina) includes the serpentines; 

 Group B (rich in lime and alumina) includes the "greenstones," 

 hornblende schists and gabbros — rocks which, however much they 

 may differ in appearance, have considerable resemblance in their 

 fundamental constitution. 



