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



Water i h yg rometric * 38 



* atcr { combined 3-93 



Silica 36-74 



Carbonic anhydride 9*10 



Phosphoric „ -35 



Alumina 17*55 



Ferric oxide 1-62 



Ferrous oxide r 8 "23 



Manganous oxide . , . . . trace 



Lime 12-08 



Magnesia 6-04 



Potassa -74 



Soda 3-08 



99-84 



Analysis shows that in its present state this rock contains a large 

 proportion of carbonate of calcium ; and since the lime found is only 

 5 per cent, in excess of that required by the carbonic anhydride, it 

 follows that the original doleritic material must haye undergone 

 extensive alteration. 



A section from this locality is represented in fig. 3 (Plate XXI.), in 

 which a group of felspar crystals, magnified 22 diameters, is seen in 

 the centre of the field ; below these are numerous cavities filled with 

 calcite, while above them are stains of ferric oxide. The sanidine- 

 like crystals are generally rendered cloudy by products of alteration, 

 and are often divided by fissures which have become filled with 

 calcic carbonate. Less frequently the structure of altered felspar 

 crystals has become to some extent spheerulitic, from the rearrange- 

 ment of their constituents. These spheroidal bodies, of which the 

 largest are about *33 millim. in diameter, resemble minute agates, 

 being formed of alternate amorphous and slightly crystalline bands, 

 while their centre is occupied by a transparent substance, into which, 

 when seen in polarized light, minute crystals are observed to con- 

 verge. 



Fig. 4 (Plate XXL), represents a portion of an altered felspar 

 crystal magnified 14 diameters. Although possessing a distinctly 

 schistose structure, this rock is evidently a lava in which the vesicles 

 are flattened parallel to the planes of foliation. 



North-eastern District. — On Treglynn Common is found a rock 

 which in hand specimens appears to be made up of a felted mass 

 of longitudinally striated crystals 5 millims. in length, and of about 

 half that width, of a dark green mineral, possessing a somewhat 

 bronzy lustre. Under the microscope this material, which consti- 

 tutes nearly two thirds of the rock, is found to be distinctly dichroic, 

 and to be often composed of parallel hornblendic belonites, some- 

 times terminating in a brush at the extremities, where the belonites 

 are frequently more or less curved. In other cases the character of 

 these imperfect crystals is indicated by parallel lines in the direction 



