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



I. II. 



~ , f hygrometric *32 *30 ■ 



^ ater {comined 2-02 2-00 



Silica, 46-42 46-37 



Carbonic anhydride 2-32 2-31 



Phosphoric „ -98 -85 



Alumina ' 20*23 20-42 



Ferric oxide 4*43 4-45 



Ferrous oxide 5-89 6-02 



Manganous oxide trace trace 



Lime 5-99 6-08 



Magnesia 3-82 3*87 



Potassa 1-95 1-83 



Soda 5-00 5-09 



Lithia trace trace 



99-37 99-59 



The coarse-grained variety of this rock consists largely of felspar 

 crystals, of which a considerable proportion exhibit, when seen in 

 polarized light, the striation of plagioclase. In some 'cases, how- 

 ever, the stria? are either indistinct or entirely absent, while not 

 unfrequently the felspar has become merged into a granular mass 

 enclosing patches of calcite and quartz. Throughout this ground- 

 mass are disseminated flakes of brown mica, some hornblende, and 

 viridite, with grains of magnetite and an unusually large amount 

 of apatite. Mixed with these are numerous small crystals of epidote 

 of a pale yellow colour, which are moderately abundant throughout 

 the mass. 



The darker rock from this quarry is much finer in grain than 

 the above, and differs from it only inasmuch as the felspar 

 has generally become decomposed, while mica is more abundant, 

 and a few small patches of some partially altered augitic mineral are 

 present. 



The joints and headings which occur in the altered doleritic rocks 

 of the neighbourhood of Saltash, instead of being filled with a brown 

 ochreous clay as in "Western and Central Cornwall, are usually 

 coated with a layer of crystalline calcite. 



" Dunstones." — Yesicular lavas are plentiful in South-eastern 

 Cornwall ; but they differ in no respect from those which have been 

 already described as occurring along the northern coast of the 

 county, excepting that their vesicles are more constantly filled with 

 calcite than they are in similar rocks found further north. 



Here, as well as in the neighbourhood of Port Isaac, this material 

 is sometimes employed for repairing the roads ; and quarries have in 

 various places been opened upon it for obtaining stone for this pur- 

 pose, as at Polbissick, near Landrake, immediately south of the road 

 at Burrel, and elsewhere. A flow of lava of considerable thickness 

 has also been cut through in making the road near Callington New 

 Bridge, and affords an instructive section. 



