J. H. Collins — Cornish Serpentinous Rocks. 



363 



near the ancient earthwork called Trethullan Castle, in the parish of 

 St. Mewan (near St. Austell), to the sea near Duporth, a distance of 

 over four miles. The typical greenstone in the St. Mewan quarries 

 often contains asbestos and thin layers of serpentine in its joints, and 

 an examination of this intrusive mass in different parts of its course 

 shows that it becomes more coarsely crystalline and porphyritic as it 

 approaches the cliffs, and that the chemical composition is also 

 changed, until finally the hornblendic rock of St. Mewan becomes a 

 serpentine at Duporth. 



The less decomposed portions near the sea-level are seen to consist 

 of serpentine of a darker green colour, full of greyish- white spots of 

 a substance resembling kaolin, which on examination appear to be 

 pseudomorphs of hornblende and perhaps felspar, varying from -£$ 

 to j of an inch in length. The crystals have not quite lost their 

 power of depolarizing polarized light, but the difference between the 

 crystals and the green base in this respect is not extremely great. 



The green serpentine base is often somewhat fibrous, the fibres 

 curving round the larger crystals and exhibiting brilliant colours 

 with polarized light ; it is sprinkled with very small prisms of 

 scarcely altered felspar. The base and the altered included crystals 

 are both sprinkled pretty thickly with crystals and grains of oxide 

 of iron, and the whole rock is in places stained yellow or brown 

 with per-oxide of iron. 



The following analyses show the interesting changes of chemical 

 composition exhibited in this mass in different parts of its course, 

 (a) is the mean of three analyses of the rock at St. Mewan made by 

 Mr. Phillips, 1 (b) is my own analysis of the rock from the quarry 

 near Gewans, (c) my analysis of the rock at Duporth, and (d) a 

 recent analysis of the same rock made by Mr. J. Arthur Phillips. 2 

 (e) is a partial analysis of a selected sample which is more free from 

 included crystals and from oxides of iron than ordinarily. 



Sp. Gr 



a. 

 2-27 .. 



•80 .. 

 47-57 .. 



•12 .. 

 17-16 .. 

 13-041 

 9-30) - 



4-11 .. 



2-301 



5-45) •• 



b. 

 . 2-86 . 



•80 . 



. 50-24 . 

 . trace 



■20 . 

 . 19-19 



. 15-30 . 



. trace 



trace . 



. 5-02 . 



•93 . 



. 7-21 . 



c. 

 2-64 . 



.. 8-65 . 



.. 37-09 . 

 . . trace 



•21 . 



.. 19-90 . 



( 15 -54 . 



■• \ 2-02 . 



trace 



.. trace 

 .. 15-90 . 



. . trace 



d. 



.. 2-86 .. 



( -721 

 •• ( 9-29/ •• 

 .. 35-74 .. 



•is .'. 

 .. 12-23 .. 

 .. 4-681 

 .. 13-84) ■• 



•98 .. 



trace .. 



.. 22-13 .. 



trace .. 



•25 .. 



e. 



Water 



. 11-5 



Titanic Acid 



. 38-0 



Phosphoric Acid 



Ferrous Oxide 



'. 4-3 

 . 8-8 



Sulphur 



. 



Lime 







Magnesia 



. 29-2 



Soda 



. ■ 







99-85 



98-89 



99-31 



100-04 



91-08 



1 Phil. Mag. Feb. 1871. 



2 For this analysis, which was given in the Mineralogical Magazine, loc. cit., I 

 am indebted to Mr. Phillips. He remarks, " I find more magnesia than you do, but 



