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ON A SODA-FELSPAR ROCK AT DINAS HEAD, NEAR 

 PADSTOW, 



Br HOWARD FOX, F.G.S., President of the Royal Geological Society of Cornwall, 

 and one of the Council of the R.I.C. 



An interesting rock occurs at Dinas Head, four and a half 

 miles west of Padstow. This rock is interbeided with slates 

 and also in immediate contact with greenstone. It weathers 

 white and has the appearance of chert, varying in colour from 

 creamy grey to various shades of brown and dark bluish grey. 

 In many places it is studded with cavities, not unlike pholas 

 holes, which have been filled with rusty coloured material 

 containing crystallised quartz. This rusty material occasionally 

 weathers out as nodules three-eighths of an inch in diameter, 

 projecting, from a quarter to half an inch, beyond the white 

 surface of the rock. Most of the rock shows distinct bedding 

 and even lamination ; but in a limited area near the N.E. corner 

 of the peninsula, its entire substance assumes a nodular grey 

 character weathering grey, and some of the nodules have a 

 radiated or spherulitic structure. 



Mr. Beringer having kindly undertaken the analysis of 

 three specimens of this rock, representing its three chief varieties, 

 entrusted the work to his pupil, Mr. A. F. Hosking, who 

 determined the rock to be similar in composition to a soda 

 felspar or albite, containing nearly ten per cent, of soda, and a 

 slight per centage of potash. The rock, however, does not 

 show the optical characters of albite under the microscope. The 

 compact varieties are crypto-crystalline, and might easily be 

 mistaken for chert ; but the concretionary and spherulitic 

 varieties show grains and blades of a felspar which is doubtless 

 albite. 



Some unweathered pieces were subsequently analysed by 

 Mr. J. Hort Player, of London, who confirmed the results 

 arrived at by Mr. Hosking. The silica in the various analyses 

 varied from 64'4 to 66 6; the alumina from 19-6 to 20-2 per 



