OF CENTRAL AND EASTERN CORNWALL. 475 



Near Tregorrick, a little east of the Pentewan valley, a quarry is 

 worked upon a " blue elvan," very closely resembling that raised near 

 the Sanctuary fields, but usually somewhat less hard and often 

 more slaty in its structure. Under the microscope it is observed to 

 have undergone extensive alteration ; triclinic felspar is still, how- 

 ever, distinguishable, and patches and microlites of hornblende are 

 abundant ; but augite has entirely disappeared. There is a little, pro- 

 bably dissociated, silica present in the form of quartz granules ; apatite 

 is observed in considerable quantities ; and pseudomorphs after mag- 

 netite or ilmenite are abundant * ; crystalline chlorite sometimes 

 occurs in the partially kaolinized felspar. 



On the sea-coast at Hallane Mill, near the Blackhead, there is 

 another outcrop of a rock of nearly similar character ; but it has in 

 some respects undergone less extensive metamorphism, since it still 

 retains a considerable amount of an unaltered augitic mineral. 

 Sections obtained from this locality mainly consist of augite, horn- 

 blende, a very subordinate amount of plagioclase, a little kaolinized 

 felspar, and peroxidized and hydrated magnetite, apatite, and quartz ; 

 the latter encloses well-defined crystals of chlorite, and occasionally 

 contains minute liquid- cavities. 



A quarter of a mile east of this point the doleritic rock is inter- 

 sected by what at first sight appears to be an elvan dyke. A micro- 

 scopical examination, however, of thin sections shows that this 

 apparent dyke is a vein chiefly consisting of yellowish quartz con- 

 taining fragments of the enclosing rock and patches of pyrites, 

 together with a few crystals of mispickel. 



East of this point the igneous rocks of the Blackhead proper are 

 for the most part much altered. They contain a considerable amount 

 of quartz, crystals of triclinic felspar (many of which have become 

 partially obliterated and merged into an amorphous mass), a large 

 amount of magnetite, a few long needles of apatite, and interlaced 

 nearly colourless hornblendic microlites. The " greenstones," of which 

 a considerable portion of this promontory is composed, have not 

 been observed to contain any unaltered pyroxenic mineral; there 

 can nevertheless be little doubt that, like those of the Sanctuaries, 



amounts of alumina and magnesia, and in the relative proportions of ferric 

 and ferrous oxide found. 



An examination of a specimen operated upon by Mr. Collins shows that 

 it has undergone a change, differing somewhat in character from that pre- 

 viously analyzed in my laboratory, as it is not only harder and darker in 

 colour, but also encloses a considerably less proportion of what would appear 

 to be altered crystals of felspar. 



* When the black crystalline grains which occur in rocks belonging to this 

 class become transformed by alteration into an opaque greyish-white substance, 

 the mineral is generally mentioned by petrographers as ilmenite. Many of the 

 crystals so altered in Cornish rocks would appear to belong rather to the first 

 than to the third crystalline system ; but their outlines in thin sections are so 

 dependent upon the direction of the plane in which they have been cut 

 through, that their identification is often difficult. There is also* reason to 

 believe that a large proportion of the magnetite in such rocks is titaniferous. 



