OP CENTRAL AND EASTERN CORNWALL. 479 



A quarry has been opened on a band of crystalline rock of a 

 green colour, at Tregellan, in the parish of St. Mabyn. This con- 

 tains no augite, but a considerable amount of hornblende and 

 viridite, together with flakes of dark brown mica, apatite, and 

 crystalline patches of partially replaced magnetite or ilmenite. 

 A few minute crystals of a yellowish mineral, which is probably 

 epidote, are also present; these are often arranged around frag- 

 ments of ilmenite or magnetite as a centre. A large proportion of 

 the felspar is triclinic ; but in some places it is so far decomposed as 

 to merge into an almost amorphous mass. 



At Trebisquite, in the same parish, a slaty rock occurs which 

 has evidently undergone a large amount of alteration. When 

 sections from this locality are examined, they are found to consist 

 of a colourless transparent base, enclosing patches of viridite, 

 through which are disseminated reticulated microlites of a greenish 

 colour. A large amount of much-altered ilmenite is also present ; 

 but no well-defined crystals of apatite could be distinguished. If 

 felspar was an original constituent of this rock, it has become com- 

 pletely metamorphosed. 



Near Highgate, St. Mabyn, there occurs a band of nearly similar 

 rock. This is even more slaty than the foregoing, and has suffered 

 still more extensively from alteration. A microscopical examination 

 of thin sections from this place shows that the rock is composed of 

 a granular transparent ground-mass, in places stained by hydrated 

 ferric oxide, and that it encloses numerous granules of quartz with 

 a few indistinct greenish microlites, together with a little altered 

 magnetite. This rock appears to have been originally vesicular, the 

 cavities being now filled either with calcite or with a mixture of 

 that mineral with granular quartz and viridite. 



Half a mile east of the parish church of St. Tudy a quarry has 

 been opened in a wood immediately above Penrose Cottage, on the 

 Wadebridge road, where a bluish-grey trap is somewhat exten- 

 sively worked as a material for road-making. Although of eruptive 

 origin, this rock is distinctly foliated. An examination of thin 

 sections shows it to consist of a ground-mass mainly composed of 

 plagioclase, viridite, and green hornblende, through which are dis- 

 seminated numerous minute garnets, a considerable amount of 

 titan of errite, a little apatite, and occasional grains of quartz. 



A much-altered doleritic rock is quarried near Michaelstow 

 Beacon ; and a band of greenish-blue slate has been worked for 

 road-material at Helsbury, a little east of the Camelford road. 

 This slate has a ground-mass largely consisting of granular quartz, 

 enclosing a few minute flakes of brown mica, with a nebulous grey 

 mineral and occasional greenish belonites ; it also encloses grains of 

 iron pyrites. 



A hundred yards west of the main street at Camelford a hard 

 cherty grey rock is worked for road-metal at a point known as 

 " Hill Head." A freshly broken specimen from this locality afforded 

 by analysis the following results (sp. gr. = 2-92) : — 



Q. J. G. S. No. 135. 2 k 



