OF CENTRAL AND EASTERN CORNWALL. 489 



immediately north of the turnpike road. Hand-specimens of this 

 rock closely resemble that from Hendra Chapel, and appear to be 

 chiefly composed of a mineral exhibiting, although in a far less de- 

 gree, the iridescence of bastite. Under the microscope thin sections 

 are seen to be composed of a green monochroic ground-mass largely 

 consisting of viridite, in which slender crystals of hornblende are 

 arranged in stellate and bacillar groups ; it also encloses patches of 

 green hornblende distinctly exhibiting the characteristic angular 

 cleavage- striations of that mineral. In this matrix are included 

 crystals of sahlite (?), a little magnetite, and a few minute crystals 

 of epidote. 



Smeaton Quarry is situated three miles south-east of Callington, 

 and is worked upon bands of an exceedingly hard stone, generally 

 of a greyish-green colour, which dip towards the south. Inter- 

 bedded with the greenish rock are bands of a much lighter colour, 

 which are considerably less hard than the darker variety. The darker 

 rock, although in hand-specimens almost identical in appearance with 

 the " hornblende-slate " of Mr. Rogers, is found, on examination of 

 thin sections, to be of sedimentary origin, and to consist of a quart- 

 zose ground-mass, through which is disseminated a greenish fioccu- 

 lent material, together with sericite, the whole being occasionally 

 stained by spots of hydrated ferric oxide. This ground-mass 

 encloses numerous rounded grains of quartz, having on an average 

 a diameter of *16 millim. When examined in polarized light, these 

 are seen to be each made up of numerous angular granules. 



The lighter-coloured rock does not materially differ from the 

 other, excepting that it contains but little of the flocculent 

 greenish matter, and that the included granules of quartz are more 

 angular and less numerous. 



A little west of Pounder a quarry is worked on a rock which, 

 with the exception of containing patches of quartz and a few grains 

 of calcite, is composed of crystalline augite, plagioclase, and grains 

 of magnetite or ilmenite. 



In a field adjoining the main road, a hundred yards north of the 

 church at Lan drake, is a quarry, now abandoned, from which a trap 

 of the usual character was formerly obtained. It was found some- 

 what difficult to procure freshly-broken specimens from this locality ; 

 but sections prepared from those collected show that they consist of 

 more or less altered plagioclase, augite, viridite, ilmenite and a few 

 hornblendic belonites ; a greyish flocculent product of alteration is 

 disseminated throughout the mass. 



A short distance west of the turnpike-road, near Hatt, is a quarry 

 from which stone is obtained for the repair of the neighbouring 

 roads. This rock, which is of the usual colour, is somewhat slaty 

 in texture, and is rather coarse-grained than otherwise. Under the 

 microscope the felspar, which has not generally suffered extensive 

 alteration, is seen to be almost wholly triclinic, while the augite has 

 experienced a species of decomposition by which it has been rendered 

 opaque, excepting along certain irregular lines. This is occasioned 



