NEIGHBOTJRHOOD OF ST. MIXVER, CORNWALL. 397 



When we examine a section of this rock under the microscope, 

 we find that some of the original glassy basis has to a large extent 

 been converted into serpentine, especiall}' along lines which seem to 

 indicate fluxion-structure. How far this process of alteration has 

 taken place in the glass it is difficult to determine. There may be 

 a little unaltered glass still remaining ; but this is a doubtful point. 

 The glassy or once glassy portions of the rock are of a yellowish- 

 brown or coffee-colour, and are full of vesicles which are frequently 

 filled with chalcedony and often give the dark cross common to 

 radiate crystallizations when viewed between crossed nicols. There 

 is, in addition to the serpentine, a large proportion of kaolin present. 



Nos. 5 and 6. Cant Hill, East side, at foot of hill, not in lolace. — 

 In sections nos. o and 6, cut from specimens collected from debris 

 on the shore, on the east side of the hill, nearly opposite the slate- 

 quarry, we find the same vesicular lava, devitrified and in a more or 

 less crushed condition*, with a very considerable admixture of 

 calcite, serpentine, and some felsitic matter. Quartz often occurs 

 in the vesicles. There seems, then, no doubt that the up])er portion 

 of Cant Hill, which has been mapped as " greenstone," is really 

 composed of a basic lava of a once vitreous character, but so altered 

 that its original mineral constitution cannot be inferred with 

 precision. Whether the patch mapped as greenstone on the western 

 side of the estuary, stretching from Oldtown Cove to Tregouy, and 

 the exposures on the east of Cant Hill, touching Tregenna and 

 Dinham House, are extensions of this lava of Cant Hill, T am unable 

 to say, as I did not visit those localities; but, judging from their 

 relative positions on the map, it seems by no means an improbable 

 supposition. 



No. 7. Carlion, near St. Minver. — These specimens were collected 

 in a small quarry on the top of a hill situated close to Cant Hill, a 

 road leading down from this quarry to Cant Farm being at the time 

 mended with some of this freshly broken stone. 



It is a compact, greyish-green rock, with dark green or greenish- 

 black blotches, which, when it is roughly broken for road-metal, look 

 at first sight like rain-spots on an absorbent stone. Here and there 

 these spots show glistening cleavage-planes. 



Under the microscope the constituents are seen to be augite, 

 triclinic felspars (apparently labradorite), ilraenite, a considerable 

 amount of felsitic matter, a little chalcedony, and, in some cases, 

 serpentine. The most striking features of the sections are the crys- 

 tals of augite. These are seen to be broken up into exceedingly 

 irregular patches, in places somewhat like the pieces of a dissected 

 puzzle, loosely shuffled together or slightly pushed apart (fig. 2, 

 p. 398) ; this is due to the augite crystals ha\dng enveloped small 

 felspar crystals and felspathic portions of the rock during the process 

 of crystallization, and not to any corrosive action. 



In most cases the envelo])ed felspars have been converted into 

 felsitic matter, a fate which many of the larger felspar crystals have 

 also shared. 



* Prof. Bonney is strongly inclined to regard no. (> as an ash. 



