J. H. Collins — Cornish Serpentines. 301 



tially or wholly replaced by magnesia " (Geol. Hist, of Serpentines, 

 1873, p, 169). These experiments date from 1860, their accuracy 

 has never, I believe, been disputed, and the hypothesis, as regards 

 the formation of serpentine, " has been adopted by Delesse, Giimbel, 

 Credner, Favre, Dieulefait, and Stapf " (ibid. p. 206). 



As regards the, removal of alumina in solution, which has been 

 thought to be a difficulty. Dr. Heddle says, "Silicate of alumina can 

 be decomposed by chloride of magnesium or sulphate of magnesium, 

 silicate of magnesia being formed " (quoted in Min. Mag. vol. iii. p. 

 139). I myself have found that sea- water, aided by heat and pres- 

 sure dissolves out not onl}'- lime (when present) and alkalies, but 

 also alumina and a little silica from all varieties of felspar. 



The late David Forbes was a firm believer in the dolomitic origin 

 of much of the Lizard serpentine. The following are two of his 

 analyses of serpentinous rocks fi'om a Cornish locality as given by 

 Capt. Marshall-Hall (Min. Mag. vol. iii. p. 213) :— 



Dolomitic. Serpentinic. 



Silica 



3-43 

 Mo 



39-48 



Oxides of iron and alumina 



1-65 



Lime 



13-37 



24-14 



Magnesia 



20-12 



18-16 



Carbonic acid 



49-04 



13-83 



Water 



12-91 



4-56 



100-02 101-82 



Of course these substances are much farther removed from true 

 serpentines than those ordinarily met with in the Lizard district. 

 The changes indeed are evidently far from complete; they may nevei'- 

 theless serve in some degree to indicate one mode in which such 

 transformations have been brought about. 



The origin of the Porthalla serpentine, as well as that of Mullion 

 and the greater part of the Lizard district has been, I believe, 

 brought about pretty much as follows : — 



A series of beds of ancient stratified rocks — including siliceous 

 limestones and dolomites of varying composition and texture, some 

 therefore, more permeable than others — has been contorted and com- 

 pressed during many ages. At some periods during the long series 

 of changes, they have been subject to the action of magnesian 

 chlorides in solution, at an elevated temperature, and under consider- 

 able pressure ; submergence in the waters of the sea to a con- 

 siderable depth would suffice to give all these conditions. 



In this way from the more permeable beds silicates of alkalies and 

 of lime together with alumina might be carried away in solution and 

 silicates of magnesia deposited — the oxides of iron present remain- 

 ing unchanged or nearly so. The less permeable beds perhaps have 

 suffered internal changes of a less complete character — the ultimate 

 chemical composition here may perhaps be pretty much as it was 

 originally, but the components have been rearranged so as to give 

 rise to the peculiar hornblendic and felspathic constituent minerals 

 now present. Of course if there had been any veins of igneous 

 rock previously injected before these chemical agencies had got to 



