336 J. ARTHUR PHILLIPS ON THE ROCKS OF 



On comparing the first three of these analyses with one another 

 and also with those of Cornish granites (p. 330), their closely ap- 

 proximate uniformity of composition will at once become evident. 



I. From Pra Sands, near Sydney Cove. — This elvan, which is in 

 killas, runs north of east to Tregurtha, where it turns southward, 

 passing through St. Hilary. At Tregurtha it sends off a branch in 

 a south-easterly direction, which enters the sea at Pra Sands, where 

 it is about 12 fathoms in width. Towards the centre of this dyke 

 the rock is highly porphyritic, consisting of a mixture of quartz with 

 white and pink felspar, imbedded in a brownish -red felspathic base ; 

 the crystals of felspar are sometimes from two to three inches in 

 length. It also contains mica, a little schorl, and occasional crystals 

 of pinite. Under the microscope it is seen to consist of a felspathic 

 base, through which indistinct scales of a greyish green colour are 

 thickly sprinkled, and enclosing porphyritically imbedded crystals of 

 quartz and orthoclase, with a few flakes of mica and a little schorl, 

 sometimes much altered. The quartz has frequently caught up 

 portions of the felspathic base, giving rise to stone-cavities, similar in 

 general appearance to the glass-cavities described by Mr. S. Allport 

 as occurring in the pitchstones of Arran*. The large crystals of fel- 

 spar are orthoclase, and sometimes enclose quartz, numerous needles 

 of schorl, and flakes of mica. Under a high power the quartz 

 is seen to enclose hair-like crystals of schorl, some of which are 

 broken, together with gas- and fluid-cavities : some of the latter 

 contain no bubble ; while others appear to be more or less coated 

 with clay, as though muddy waters were present at the time of their 

 formation. 



II. From Tregoning Hill, Breage. — This elvan occurs in the form 

 of an east-and-west dyke in the granite of Tregoning Hill, and is 

 a somewhat fine-grained rock composed of a grey felspathic base, 

 porphyritically enclosing a few small crystals of felspar and grains 

 of quartz, together with numerous six- and twelve-sided prisms of 

 pinite ; schorl is also sometimes present in small quantities. 



Thin sections of this rock do not exhibit under low powers any 

 characteristics which are not recognized when an ordinary speci- 

 men is examined by the aid of a lens ; but when a 'combination 

 magnifying 350 linear is employed, the light nebulous grey base 

 is seen to be cryptocrystalline, and to enclose a few distinct crys- 

 tals of monoclinic felspar, a very small quantity of quartz with 

 schorl and chloritef, pinite in greater abundance, and a few flakes 

 of mica. 



III. From Mellanear, near Hayle. — A very fine-grained elvan, 

 occurring in killas, forming a dyke 40 fathoms in width, bearing 

 north of east. Its general colour is a dull bluish grey ; it encloses 

 a few distinct crystals of white felspar, and occasionally small nests 



* Geol. Mag. vol. ix. 1872, p. 536. 



t Iii this and many other varieties of Cornish elvan the greyish green 

 nebulous matter, which has been described as pervading to some extent the base 

 of so many different rocks, is seen to become gradually condensed into vermicu- 

 lar aggregations of what appears to be chlorite. 



