Vol. 59.] GRANITE AND GEEISEN OF CLIGGA HEAD. 155 



the solidification of the rock, however, in the Tertiary granophyres 

 of the "Western Isles of Scotland and the North of Ireland. 1 



The greisen of Cligga Head differs from that of St. Michael's 

 Mount in the absence, so far as has yet been ascertained, of apatite, 

 which would denote the presence of phosphoric acid among the meta- 

 somatizing reagents. Nevertheless, it is not unsafe to predict that 

 apatite will be found at Cligga Head, for Prof. Le Neve Poster 2 has 

 recorded that mineral from the lodes of Wheal Kitty, situated 

 between the Cligga and St. Agnes granite-outcrops ; and for the 

 same reason it may also be fairly expected that fluorspar will be 

 found. 



When at St. Agnes, I was told by two inhabitants of considerable 

 experience that garnet had been found at Cligga Head, and was 

 shown a specimen from that locality. Although not fortunate 

 enough to find any specimens in situ, I do not hesitate to accept this 

 statement, and would mention in support of it that Mr. Whitman 

 Cross 3 records sanidine and garnet with topaz in the lithophyses of 

 rhyolites from Colorado. 



It has already been stated that in the granite-tongue the greisen- 

 bands become very thin and frequent, and at the extremity appear 

 to die out altogether in some cases. More than this, other modifica- 

 tions make their appearance ; for veins of orthoclase and of fresh 

 porplryritic biotite-crystals, some half an inch across, diversify the 

 structure of the rock. With regard to the biotite, it appeared to 

 me that the extremity of the tongue was considerably richer in that 

 mineral than the rest of the granite ; but owing to the altered 

 state of that mica farther north no conclusive evidence can be 

 adduced. It is a notable fact, at any rate, that the fresh con- 

 dition of the biotite here indicates that the metasomatizing reagents 

 did not affect the granite at the extremity of the tongue to such 

 an extent as that at its root or in the main mass.' 



It only remains now to describe the minerals that were found 

 lining the ' bedding-planes ' at the extremity of the tongue. The 

 actual source from which they were collected was a large fallen block 

 lying at the bottom of the path which leads up the cliff at that point. 

 Besides wolfram, which is commonly met with in the 'bedding- 

 planes ' there, topaz, lithia-mica, and a pink felspar occur. 



The topaz occurs in short prisms, rarely exceeding a quarter of an 

 inch in length, colourless or very pale green, and in one case forming 

 a continuous mass coating the face of the rock. The form of the 

 crystals is simpler than that described by Greg & Lettsom, and 

 by Mr. J. H. Collins, as commonly occurring in the Cornish topaz. 

 A typical crystal was measured and found to be made up of the 



1 For further literature on this subject, see Prof. Jucld's paper above quoted. 



2 Trans. Roy. Geol. Soe. Cornw. vol. ix (1878) p. 210. 

 • J Am. Journ. Sci. ser. 3, vol. xxxi (1886) p. 432. 



