330 



J. ARTHUR PHILLIPS ON THE ROCKS OF 



afford colours with polarized light. This is probably not an eruptive 

 rock, but a somewhat altered hornblendic slate. 



Dykes of diorite occur in the neighbourhood of St. Austell &c. ; 

 and numerous traps, the composition of which has not as yet been 

 determined, are found in different parts of the county. 



Granite. 



The granite of Cornwall is usually coarse-grained, but varies con- 

 siderably in this respect in different localities. In addition to quartz, 

 felspar, and mica, it almost invariably contains schorl, with some- 

 times, as accessories, chlorite, apatite, fluor-spar, beryl, cassiterite, 

 garnet, and pinite. 



In some districts mica is replaced by a talc-like mineral, and the 

 rock thus apparently passes into protogine. Coarse-grained granites 

 are occasionally traversed by granitic veins of a finer texture. 



Mica is sometimes almost entirely replaced by tourmaline, as in 

 some parts of the parish of Luxulyan, and near Eoche. At Eoche 

 Eock and near St. Mewan Beacon the felspar has disappeared, and 

 the rock consists of a mixture of quartz and schorl only. 



Table showing the Composition of three varieties of Cornish 

 Granite. 





I* 



II. 



III. 



™ , f hvsrrometric 



•34 



•89 



74-69 



16-21 



116 



trace 



•58 



•28 



•48 



3 64 



1-18 



•10 



•87 

 trace 

 74-54 

 14-86 



•23 

 2-53 

 trace 



•29 

 trace 

 3-73 

 3-49 

 trace 



•33 



•89 

 7065 

 16-16 



■52 

 1-53 

 trace 



•55 

 trace 

 8-66 



•54 



Water { combined t 





Alumina 





Ferric „ 



Manganous oxide 



Lime 



Magnesia. 









99-55 



10054 



99-83 



2-64 



2-66 



2-62 





The felspar of Cornish granites, which has often undergone a con- 

 siderable amount of decomposition, is for the most part orthoclase ; 

 but a plagioclastic species, stated by Professor Haughton to be albite, 



* Phil. Mag. 1873, vol. xlvi. p. 30. 



t The whole of the water retained by crystalline rocks, after they have ceased 

 to lose weight at 100° C, cannot exist in a state of combination. A portion of 

 it is evidently enclosed in the fluid-cavities, vast numbers of which must escape 

 being crushed during the grinding of the rocks for analysis. 



