A. R. Sunt — Devonian Rocks of South Devon. 343 



sented at Bickerton, a large accession of magnesia is requisite ; and 

 for the carbonates a considerable amount of carbonic acid. 



Hydro-thermal and dynamic action combined seem to meet the 

 case, provided that the super-heated water be sea- water charged with 

 carbonic acid. An instance of hot salt water, probably sea-water, 

 giving rise to chlorite, brown hornblende, pale-green actinolite, and 

 pyrites is recorded by the late Mr. J. A. Phillips in the case of the 

 Huel Seton Mine.^ Mr. Ussher has sent me a specimen of an acid 

 eruptive rock, occurring within a mile and a half of the metamorphic 

 area, whose quartzes are charged with fluid carbonic acid ; so we 

 have evidence of volcanic action, with abundance of carbonic acid, 

 in the neighbourhood of the green rocks. 



With magnesia and carbonic acid available from an outside source 

 the process of metaraorphism seems easy. 



Starting with augite and lime-soda felspar, we have (leaving out 

 silica which is common to all the original minerals concerned ; and 

 iron, which is common to all except felspar) : — 



Magnesia, lime , n • Alumina, masrnesia, lime 



— ~ ! 4- Alumina = , , . , — 



augite nornbienae. 



Alumina, magnesia, lime , ^ t, ■ -j Alumina, masrnesia , Carbonic acid, lime 



xT — xX J + Carbonic acid = ,, .,'' + r-ir^ 



hornblende cniorite calcite. 



Again, 



Alumina, lime-soda , r, t_ ■ -j Alumina, soda , Carbonic acid, lime 



-^ -I- Carbonic acid = ^ + ^.- — 



lelspar albite calcite. 



Alumina, lime-soda , /-, i • -j , 



^ + Carbonic acid + magnesia = 



lelspar 



Alumina, lime, magnesia , Aiu-j-p i Alumina, magnesia , rjg^^pj^g 

 hornblende chlorite 



Thus carbonated sea- water acting on augite associated with a lime- 

 soda felspar, supplies all the materials for the formation of the 

 secondary materials, hornblende, chlorite, felspar and calcite, which 

 form the mass of the Green Eocks. 



The epidote of the Green Rocks seems to be often associated with 

 a water-clear felspar, entirely free from decomposition and possibly 

 albite. The decomposition of a lime-soda felspar such as labradorite 

 would account for the association of albite with epidote, e.g. 



Alumina-lime-soda _ Alumina-soda , Alumina-lime. 



Labradorite. albite. epidote. 



There seems weighty evidence in support of the hypothesis that 

 the metamorphic rocks of S. Devon are Devonian, and of about the 

 same age as the sandstones and slates with Pleiirodicfyum frohlema- 

 iicum of the northern shore of Torbay ; in which case the genesis 

 of these Devonian schists should prove an attractive problem for 

 chemists and petrologists. Should the schists however ultimately 

 prove to have no connection with the Devonian rocks, the two 

 together will surely afford an unparalleled example of undesigned 

 coincidences. 



1 Phil. Mag. ser. 4, vol. xlvi. pp. 30, 31. 



