118 



fibres is strongly marked, though not absolutely perfect. The long axes of 

 the fibres lie approximately parallel with the vertical axis of the pyroxene. 

 This mineral commonly forms the glistening crystals in many porphyritic 

 serpentines 



Next in importance to the serpentine itself are the minerals from which 

 the serpentine has been derived. The most important of these, so far as the 

 British serpentines now under consideration are concerned, are olivine, 

 enstatite and hornblende. 



Olivine. Unaltered olivine is very abundant in many localities in 

 Cornwall, as for example at Coverack Cove, Mullion Cove, Pradanack, 

 Landewednack and Kynance Cove. Every stage of the transition from a 

 rock extremely rich in olivine to one in which all traces of the original 

 mineral have disappeared may be observed. The olivine is almost colourless 

 in thin section. 



Enstatite. Unaltered enstatite is abundant in certain localities. It is 

 colourless in thin section and possesses the strongly-marked cleavage of a 

 pyroxene. Alteration commences at the surface and along the cracks by 

 the formation of a finely fibrous structure, and ends in the formation of a 

 bastite-serpentine in which the fibrous structure is usually more or less 

 preserved. (See Fig. 2, Plate I.) The large crystals with one strongly 

 marked plane of separation and a brilliant metallic lustre, which give a 

 marked porphyritic character to many serpentines, belong in all cases 

 examined by the author to bastite. Cleavage flakes which may be detached 

 'by the aid of a penknife show in convergent polarised light a negative 

 bisectrix.' 1 ' 



The following analyses by Dr. HEDDLE appear to have been made from 

 specimens of this mineral : 



i. 11. 



SiO a ... ... 38-186 ... ... 37-776 



ALO S 2^178 '. 2-123 



Cr 2 3 ... ... -276 



Fe,0 : ... ... -028 ... ... 5-069 



FeO ... ... 8-479 ... ... 2-095 



MnO ... ... -513 ... ... -<>76 



CaO ... ... 2-912 



MgO ... ... 32-418 ... ... 37-014 



K 2 ... ... 1-401 < tr 



Na 2 ... ... -065) 



H 2 14-03 16-07 



100-486 100-223 



(1) Bastite is distinguished from bronxite by this character. In the latter mineral the 

 plane of easy cleavage is parallel to the optic axial plane, and cleavage flakes give, therefore, 

 no definite optic picture. Diallage may be easily distinguished from either of the above 

 minerals by the fact that cleavage flakes show an optic axis somewhat oblique to the axis of the 

 instrument. 



