162 



J. A. PHILLIPS ON THE SO-CALLED 



Table sTiowing the Composition of Four Varieties of Altered Crys- 

 talline Rock from tlie Penzance District*, 



Water / hygronietric 



Water 1 combined 



I. 



II. 



III. 



IV. 



•24 



•76 



46-32 



•52 



18-18 



•82 



•32 



10-92 



trace 



9-32 



7-46 



2-07 



2-95 



•21 



•80 



47-22 



•95 



19-85 



1-01 



•17 



9-87 



trace 



10-18 



6-30 



112 



2-13 



•22 

 1-17 



43-48 



distinct trace 



18-60 



3-68 

 traces 

 11-38 

 trace 

 12-31 



6-01 



1-12 



1-69 



•29 

 1-92 



47-26 

 •33 



21-64 

 3-97 



traces 

 8-92 



trace 

 6-01 

 4-02 

 1-91 

 377 



Silica 



Phosphoric anhydride 



Alumina 



Ferric oxide 



„ persulphide 



Ferrous oxide 



Manganous oxide .... 



Lime 



Magnesia 



Potassa 



Soda 





99-88 



99-81 



99-66 



100-04 



Specific gravity 



302 



2-97 



< 



3-01 



2-98 





I. Rock immediately North of St. Peter's Vicarage, Tolcarn. — 

 This is greenish and crystalline, presenting, in hand specimens, the 

 appearance of a mixture of felspar and augite, in which the latter 

 greatly predominates : some of the crystals of this mineral are one 

 fifth of an inch in length ; but those of felspar are usually less dis- 

 tinct. Thin sections examined under the microscope are found to 

 contain felspar, augite, viridite, brown and green hornblende, tre- 

 molite (?), magnetite, occasional specks of pyrites, apatite, a little 

 granular quartz, and a few flakes of mica. The felspar is much 

 altered, some of the crystals having become merged in a colour- 

 less slightly opaque base ; a few, when examined in polarized 

 light, still exhibit the characteristic striae of plagioclase, while the 

 remainder are pseudomorphs, either enclosing viridite or traversed 

 by acicular crystals of hornblende. The augite, which occurs in the 

 form of large crystals and in crystalline patches, has undergone 

 comparatively little change, excepting that in some places, and par- 

 ticularly along certain lines of fissure, it has become partly con- 

 verted into either viridite or hornblende. Magnetite is sometimes 

 partially replaced, while apatite remains unaltered. 



II. Compact Crystalline Rode from Tolcarn Quarry. — Specimens 

 of this rock do not materially differ in appearance from those of the 

 foregoing, excepting that it is less distinctly crystalline, is some- 



* It must be borne in mind that the first three of these analyses are of rocks 

 from the same locality, in different stages of alteration, while the fourth is of a 

 greenstone from a distance of some three miles. In addition to their other 

 constituents the rocks described in this paper not unfrequently afford, as pro- 

 ducts of alteration, traces both of carbonic and sulphuric acids. 



