414 MR. J„ PARKINSON ON THE GEOLOGY OF [Aug. I903, 



closely speckled with small dull-white spots about -01 inch in length. 

 The matrix in which these spots are set consists of a felted mass of 

 chlorite, through which are scattered conspicuous flakes of white 

 mica, and many of ilmenite. 



Dr. Flett and Mr. Prior have kindly looked at slides and speci- 

 mens from Tregrylls and the coast south of Boscastle, containing the 

 characteristic white mineral. The following are its optical pro- 

 perties : — It is biaxial and negative, quite translucent, unaffected by 

 heating to bright redness, cleavage is absent, the refringence and 

 birefringence are low. Minute inclusions (mica and iron-oxides) 

 are arranged parallel or nearly parallel to the longer axis of the 

 mineral, and extinction measured in regard to their direction of 

 orientation is invariably oblique. Twinning on a simple plan occurs 

 here and there, but is not well marked. Mr. Prior tells me that 

 the specific gravity is 



'near that of beryl, 2*69, the grains scratch calcite and even apatite, and 

 the mineral is not decomposed by sulphuric acid, but a few grains yielded to 

 sulphuric and hydrofluoric acids.' 



The mineral is probably orthoclase. 



The Volcanic Series. 



The banks of the River Inny, near St. Clether, afford numerous 

 examples of amygdaloidal volcanic rocks of a type which is also found 

 near Davidstow, at Trewinnow, and in the South- Western Railway- 

 cutting. The typical rock of St. Clether consists of flakes of chlorite 

 and mica, of opacite, and iron -oxides set in a translucent ' base/ 

 which assumes a finely speckled appearance between crossed nicols. 



Comparison of numerous thin sections show this to be partly 

 felspar, partly quartz. In this ' base ' yellowish granules, probably 

 of epidote, have formed, with sphene, an almost colourless horn- 

 blende, magnetite, and probably leucoxene in some sections. Lath- 

 shaped crystals of a plagioclase are not rare. 



A similar rock (though differing in appearance macroscopically) is 

 characteristic in appearance and structure. Markedly foliated, 

 slabby in fracture, and of a greenish-grey colour, it is especially dis- 

 tinguished by dark-green oval spots, which the microscope shows to 

 be amygdaloids flattened by pressure. The body of the rock consists 

 of chloritic needles and flakes, sometimes densely aggregated, some- 

 times opening to show a small grain of quartz or secondary felspar, 

 but a considerable proportion of this pale flesh-coloured base is 

 isotropic. Opacite, magnetite, etc., are disseminated evenly enough 

 throughout, and rutile occurs as a rare accessory. The amygdaloids 

 are composed of quartz, chlorite, chalcedony, and magnetite. In 

 many localities this rock is associated with a hard ashy-looking 

 slate, containing lenticular black patches of magnetite-dust — locally 

 titaniferous, the particles sometimes densely aggregated to the ex- 

 clusion of foreign material, at others forming idiomorphic crystals. 

 In some slides all stages in the concentration of the mineral may 

 be traced, from a cloudy dissemination to a dense mass (as, for 

 example, south-east of Treglasta). 



Rocks belonging to the Volcanic Series are found to the south of 



