SOME OF THE ROCKS OF THE LIZARD DISTRICT. 925 



Group A. The Serpentines. 



These rocks, as usual, present an infinite variety as regards ex- 

 ternal appearance ; but there is considerable resemblance in their 

 chemical composition. A specimen has been selected for careful 

 analysis. 



Black Serpentine from " near Cadgwith" — Black to greenish black, 

 opaque ; fracture splintery ; charged with numerous crystals of a 

 highly cleavable mineral of a pale brass-colour and metallic lustre. 

 Sp. gr. of the mass 2-587. Freed as far as possible from the crystals, 

 the black matrix yields a greenish-grey powder, which adheres in 

 considerable quantity to the magnet. Composition : — 



Dried at 100° C. 



Silicates &c. undecomposed by HC1 1-37 



Iron bisulphide 0-41 



"Water (including traces of C0 2 and C ?) .... 12-35 



Silica (pulverulent) traces of Ti 2 38-50 



Alumina 1*02 



Ferric oxide 4*66 



Ferrous oxide 3*31 



Oxide of nickel 0-59 



Lime 1-97 



Magnesia 36-40 



Sulphuric acid traces 



100-58 



Two or three kinds of crystalline fragments are noticeable in the 

 undecomposed residue ; one is probably enstatite. 



As the amount undecomposed is so small, the quantity of unaltered 

 augite, diallage or enstatite, must be very trifling. Any olivine 

 still existing would of course be dissolved ; but the quantity of water 

 found, and the fact that no gelatinous silica is noticeable, preclude 

 the possibility of there remaining any considerable amount of this 

 mineral *. 



The quantity of ferric oxide is probably wholly in combination as 

 normal magnetite ; this would absorb 2*09 out of the 3-31 of ferrous 

 oxide, leaving 1*22 ferrous oxide for the hydrous silicate which 

 forms the bulk of the mass. There exists, therefore, in all proba- 

 bility, 6- 75 per cent, of normal magnetite disseminated as a fine 

 powder throughout the mass, causing the blackness and opacity for 

 which this serpentine is remarkable. 



The undoubted presence of nickel is interesting in connexion 

 with these magnesian rocks ; it may exist partly as nickeliferous 

 pyrrhotite, and partly as a base of the hydrous silicate. It has been 



* The amount shown by the microscope is extremely small (see p. 921). — T. G. B. 



