478 J. A. PHILLIPS ON THE SO-CALLED " GREENSTONES" 



of it is easily recognized as plagioclase ; hornblende occurs plen- 

 tifully in the form of interlaced microlites ; and the ilmenite is much 

 altered. It contains calcite and patches of an unaltered pyroxenic 

 mineral ; "but none of the sections examined were observed to con- 

 tain apatite. 



In the parish of St. Minver there is a comparatively coarse- 

 grained crystalline rock of a general dark green colour, containing 

 occasional crystals of felspar. The felspar has in some instances so 

 merged into a semitransparent ground-mass, that when examined by 

 polarized light no distinct crystalline outlines are apparent ; nume- 

 rous crystals of a triclinic felspar, however, are sometimes present. 

 Crystals of apatite are also of frequent occurrence ; but the ilmenite 

 has become to a large extent replaced by a mineral of a greyish- white 

 colour. In some specimens augite is plentiful ; but it has often become 

 so changed around the edges of the crystals that, if the alteration 

 were continued, they would become converted into uralite. 



Fig. 5, Plate XXII., represents a section of this rock, magnified 

 14 diameters, exhibiting the gradual alteration of augite. 



At Slade's Bridge, three quarters of a mile east of the parish 

 church of Egloshayle, a greenish grey coarse-grained trap is exten- 

 sively worked for road-metal ; this band is of considerable thick- 

 ness, and appears to be interbedded with the associated slates. 

 An analysis of this rock afforded the following results (sp. gr. 

 =2-78):— 



Water \ combined 4-96 



Silica 46-65 



Carbonic anhydride 1-90 



Phosphoric „ trace 



Alumina 19*36 



Ferric oxide 2-61 



Ferrous oxide 9*55 



Lime 4-08 



Magnesia 5*34 



Potassa *32 



Soda 4-54 



99-71 



The presence of carbonic anhydride, together with that of a con- 

 siderable percentage of combined water, indicates that this rock has 

 undergone a certain amount of decomposition. 



Under the microscope it is seen to consist of a mixture of triclinic 

 felspar which is often partially kaolinized, with viridite and a little 

 hornblende. It likewise contains patches of ilmenite, which have to 

 a considerable extent been replaced by an opaque greyish material ; a 

 few grains of quartz, and perhaps some small crystals of epidote, are 

 also present. No unaltered augite was observed in this rock. 



