474 J. A. PHILLIPS ON THE SO-CALLED " GKEENSTONES" 



certain lines of fissure, and that the result has been the production 

 of the minerals forming the cement now holding together the various 

 isolated blocks. 



An analysis made on a freshly broken fragment of this rock 

 afforded the following results (sp. gr. =2-80) : — 



Water I h yg rometric • ■ • • ■ '? 2 



waxer \ combined 9-29 



Silica .. 35-74 



Phosphoric anhydride -18 



Alumina 12-23 



Ferric oxide 4-68 



Ferrous oxide 13-84 



Manganous oxide -98 



Lime trace 



Magnesia 22-13 



Potassa trace 



Soda ^ -25 



100-04 



The white spots described as being enclosed in the grey matrix 

 are, in the aggregate, considerably in excess of the base, and are 

 usually about 3 millims. in length, evidently representing altered 

 crystals of felspar. Thin sections examined under the microscope are 

 observed to consist largely of a granular colourless material resulting 

 from the alteration of felspar, through which are disseminated a few 

 yellowish brown microlites and slightly altered grains of magnetite, 

 together with patches of a cryptocrystalline green mineral. A few 

 needles of apatite are also present. 



It will be observed that, both in appearance and in chemical 

 composition, this rock differs essentially from the altered dolerites 

 of the class found at the Sanctuaries and elsewhere ; in its general 

 structure and mineralogical composition it also differs materially 

 from those rocks*. 



* At the meeting of the British Association held at Plymouth in 1877, a 

 communication was read by Mr. J. H. Collins, entitled " On the Serpentine of 

 Duporth, in St. -Austell Bay, Cornwall." 



In this paper he gives the following as the composition of a specimen of this 

 rock : — 



Water 8'65 (of this 273 was lost below 120° C> 



Silica 37-09 



Titanic acid trace 



Phosphoric acid -21 



Alumina 1990 



Ferric oxide 1554 



Ferrous oxide 2"02 



Oxide of manganese . . . trace 



Lime trace 



Magnesia 15-90 



Alkalies trace 



99-31 

 It will be observed that the results of the two analyses differ considerably in the 



