366 J. H. Collins — Cornish Serpentinous Rocks. 



Chromic Oxide 

 Carbonate of Lime 



Lime 



Magnesia 



Soda 



Moisture 



0-14 

 2-77 

 5-04 

 5-97 

 0-84 

 10-66 



99-10 

 Polyfant. — The "potstone" of Polyfant near Launceston has long 

 been known as a very infusible and easily wrought rock, one of the 

 few Cornish rocks suitable for elaborate and highly-finished church 

 decoration, and it has been used for such purposes in most of the 

 churches of the neighbourhood, 1 but I am not aware that the rock 

 has been hitherto described from the point of view of the petrologist. 

 The quarries are very ancient and much filled in, but as far as can be 

 seen they lie in the strike of the associated slates. To the unaided 

 eye the rock appears as a dark greyish-green slightly talcose mass, 

 in some places spotted brown with peroxide of iron. Its structure 

 is somewhat schistose and at the same time brecciated. Whether it be 

 an intrusive mass or an altered volcanic agglomerate is still somewhat 

 uncertain, but I incline to the latter supposition. Under the micro- 

 scope, and using the 2-inch power, it exhibits a great many colour- 

 less crystalline masses, having two cleavages forming oblique angles, 

 which may be altered diallage or enstatite. They are slightly dichroic, 

 but quite dark with crossed prisms. There are also some nebulous 

 nearly circular or hexagonal patches which occasionally have a green 

 spot or a grain of magnetite in the centre. The ground-mass is gi - een 

 — in some parts dichroic ; in others not — it is resolvable under the 

 J-incb power into a fibrous mass of asbestiform material, which polar- 

 izes strongly using crossed prisms where thin. A great many black 

 square crystals of magnetite are present — also some patches of brown 

 oxide of iron probably resulting from the decomposition of pyrites. 

 My analysis of the rock — in duplicate — gives the following as its 

 composition : — 



a. b. 



13-16 ... 13-25 



"Water given off in dessicator *94) 



,, by ignition 12-22/ 



Silica ... 36-90 ... 34 



Alumina ... 11-80 ... 12 



Ferrous Oxide ... 3-56 ... 3 



Ferric Oxide ... 12-00 ... 9 



Lime ... 2*80 ... 4 



Magnesia ... 15-03 ... 18 



Potash ... 3-64 ... 3 



Soda ; ... trace ... trace 



98-89 99-34 



Although there are considerable differences between this composi- 

 tion and that of the Duporth rock, yet if chemical composition alone 

 be taken into account — and as a whole there is a good deal of 



1 A shallow basin or crucible of this rock has been recently found in close juxta- 

 position with a " celt mould " of hard sandy slate at Altarnum, a few miles from the 

 Polyfant quarries. It was probably used for melting the bronze used by the ancient 

 Britons. It is now in the Museum of the Royal Institution of Cornwall at Truro. 



