ASSOCIATED ROCKS OF THE LIZARD DISTRICT. 895 



a pale translucent bluish grey. Now and then there is a portion still 

 showing the colour and striping of plagioclase felspar. This altera- 

 tion is very common in the gabbros of this district. _ Some from 

 Coverack Cove, which I shall presently describe, show it in an early 

 stage ; some from Karak Clews in a later ; a specimen also from 

 this very headland, cut to shew the junction of the gabbro with 

 the hornblende schist, still retains its plagioclase in many parts 

 unaltered. A similar alteration has taken place in a gabbro which 

 I have collected from Mont Colon, in the Pennine Alps ; and I have 

 observed it in not a few other cases. I regard the mineral therefore 

 as a kind of pseudomorph, the result of the alteration of labradorite 

 or some plagioclase felspar. It is the mineral often called saussu- 

 rite, and is quite as hard as, sometimes a little harder than ordinary 



felspar *. 



The other mineral is sometimes diallage f, but in others a rather 

 dark green mineral, something resembling chlorite at first sight ; 

 microscopic examination proves this to be hornblende. The larger 

 patches are found to be composed of irregular aggregates of small 

 prismatic crystals and grains (or possibly occasionally folia) of that 

 mineral ; these are generally pale green in colour, fairly dichroic, 

 changing from a strong dull green to a sort of straw-green, and now 

 and then showing very distinctly the characteristic cleavage along goP 

 (fig. 3). On examining a series of specimens,both macroscopically and 



Yig. 3. — Hornblende in Gabbro Vein from the Balk. 



The part left white is altered felspar 



microscopically, this change, which we shall find to be very common 

 in these Cornish gabbros, is seen to take place as follows : — The ag- 

 gregated hornblende crystals form as a kind of border to the diallage 

 (fig. 7, p. 912), when the latter generally becomes rather opaque under 

 the microscope and loses its brilliant colours with polarized light, and 

 its metallic lustre with reflected light, assuming a greenish colour and 

 silky aspect. Small crystals of hornblende also appear here and there 

 in the body of the crystal, inserting themselves, as it were, between 



* See Mr. Hudleston's analysis, p. 927. 

 t See Mr. Hudleston's analysis, p. 927. 



