NOTES ON THE LIZARD ROCKS. 395 



the question, what is it the residue of ? The iron residue of 

 olivine being magnetic, could this rock have been an olivine 

 gabbro ? Tet the magnetic influence does not agree with such 

 a view, neither does the microscope reveal a particle of what 

 might be taken as traces of such as is found in the Lizard rocks 

 in general. Sphene is very abundant in every stage of change, 

 from the almost perfect mineral to a hydrous oxide of iron, 

 which finally masks all former traces of the original mineral ; 

 and it is probably due to this iron cement that the Manacle rocks 

 have so long survived others containing a lower percentage of 

 iron. The decomposition of sphene does not appear to have 

 possessed such expansive powers as olivine, therefore the rock 

 is not so fissured in the cleavage way, but faulting of the minerals 

 is more abundant than in the olivine gabbros, the faults in the 

 felspars being principally filled with anthophyllite. The fel- 

 spathic bands at Porthallow have also but slight affinity for the 

 magnet, which corroborates my former views respecting the 

 source from which they flowed. The two classes of serpentine 

 from the same locality shew different degrees of magnetic affinity, 

 according to the composition of the original matter ; the more 

 pronounced olivine serpentine closely corresponding with that 

 of the Black Head. 



Black Serpentine from the Neighbourhood of Cadgwith. 

 Fig. 15, plate D. 



A hand specimen of this rock is on the table, in colour it is 

 almost an invisible green or black ; its specific gravity, is 2*209. 

 Some beautiful grains of brown, yellow, and almost water-clear 

 olivine are yet visible. Under the microscope, the magma is 

 found to consist principally of light green serpentine, crowded 

 with magnetite ; in some spots in the thin slide the magnetite 

 is so abundant that light does not pass through it. Traces 

 of enstatite are also visible, but the network structure after 

 olivine predominates ; traces of other minerals have not yet 

 been observed. 



It has a high affinity for the magnet, which might be 

 anticipated from its general character, serpentines and somewhat 

 similar are abundant in various parts of the Lizard district. 



