Yol. 59.] GKANITE AND GKEISEN OF CLIGGA HEAD. 151 



silica is set free. The abundance of the quartz in greisen as 

 compared with that in granite is well known ; and its secondary 

 formation has also been noted in other granite-masses. For instance, 

 Prof. Bonney, in describing trowlesworthite, 1 calculates that in the 

 change from felspar to tourmaline, 43*9 per cent, of silica would 

 be liberated ; and in his paper on luxullianite the same author 

 accounted for the abundance of quartz by a similar process. We 

 may, therefore, conclude that the peculiar quartzose appearance of 

 greisen is due to the formation of secondary silica, consequent on 

 the breaking down chiefly of the orthoclase, but also of the biotite, 

 the amount derived from the latter being in fact roughly 17 per 

 cent., as will be shown later. 



In discussing the origin of the brown tourmaline in luxullianite, 

 Prof. Bonney suggested that that mineral may have been derived 

 from the biotite of the granite : in his description of trowlesworthite, 2 

 this derivation is stated as an indubitable fact. In the Cligga-Head 

 greisen there can be also no doubt whatever that the bulk, at any 

 rate, of the tourmaline has been derived from the biotite. The 

 presence of zircons alone suggests this ; the fact that the pieochroic 

 halos are preserved strengthens this suspicion ; and the argument is 

 clinched by the presence of ' transition-grains/ one of which not 

 only shows the basal fracture of tourmaline, but the basal cleavage 

 of the biotite at right angles to the tourmaline-fracture, numerous 

 zircons and other inclusions with pieochroic halos, 3 and traces of the 

 supposed yellow decomposition-product in the biotite. This grain 

 also shows the more strongly-marked absorption mentioned above : 

 indeed, we may argue that those grains which show stronger ab- 

 sorption than is generally seen in the tourmaline of this rock are 

 grains in which the metamorphosis from biotite to brown tourmaline 

 has not been completed. 



The inference which I have drawn from the study of these slides 

 is that all the brown tourmaline has been formed from the biotite ; 

 but as this conclusion might be questioned, it is well to offer the 

 following considerations in its support. 



Prof. Bonney suggested that the blue tourmaline in luxullianite 

 had been derived from the orthoclase; indeed, this may be taken as 

 an established fact. Again, apart from granite-modifications, it 

 was noted lately, while examining some slides prepared from the 

 St. Agnes tin-lodes, that it may be laid down as a general rule that 

 the tourmaline formed in the walls of the lode (capel) — that is to say, 

 in the shales and mudstones of the killas — is brown, while that 

 enclosed in the quartz of the lode is blue, 4 an arrangement which 

 is paralleled in the quartz-veins and greisen of Cligga Head. Now, 



1 Trans. Boy. Geol. Soc. Cornw. vol. x (1887) p. 182. 2 Ibid. p. 185. 



3 S. Airport, Quart. Journ. Greol. Soc. vol. xxxii (1876) p. 417, mentions 

 ' dark spots ' surrounding minute enclosures in a green alteration-product of 

 tourmaline. 



4 Exceptions to this distribution of brown and blue tourmaline undoubtedly 

 occur : for example, see Allport, op. supra cit. p. 415, & J. H. Collins, Min. 

 Mag. vol. iv (1880) p. 20. 



