148 mr. j. b. scrivenor on the [May 1903, 



the granite, caused by the falling of the cliff. These have a north- 

 and-south course ; and in addition to them are three comby 

 quartz-veins, two with a similar course to that of the joints, the 

 third trending east and west. These quartz-veins mark fissures 

 opened in the granite, subsequently to the formation of the ' bedded ' 

 structure, and need not be considered further. 



The ' bedding-planes ' are marked by bands of a dark-looking 

 rock, already identified asgreisenby Prof. Le Neve Foster. They 

 vary in thickness from an inch, or even less in the tongue, to 

 20 inches. The granite-bands, owing to the number of the ' bedding- 

 planes,' rarely exceed 20 inches in thickness, and that only in the main 

 mass — very often the} T are less than 3 inches thick. Every greisen- 

 band is traversed by a quartz-vein which occupies the fissure, along 

 which the agents whereby the metasomatism of the granite was 

 effected were conducted. The quartz-veins yield cassiterite in well- 

 formed crystals, blue tourmaline, wolfram, mispickel, and, as may be 

 inferred from the presence of decomposition-products, copper-pyrites. 

 Prof. Le Neve Foster also mentions lithomarge and lepidolite. 



In the granite-tongue the greisen-bands become very frequent 

 and thinner : indeed, at the extremity, they appear to die out 

 altogether in some cases ; yet the ' bedding-planes ' even then 

 remain well defined, and contain minerals which will be described 

 later. 



A few east-and-west faults, with a small downthrow, can be seen 

 cutting the greisen-bands as one proceeds along the beach ; they are, 

 however, of small importance, and may be dismissed without further 

 notice. 



Owing to the proximity of the ' bedding-planes,' one to another, 

 the granite is nowhere in the Cligga section sufficiently fresh to 

 warrant the preparation of slices for the study of the nature of the 

 original minerals. It may be seen from a hand-specimen that the 

 biotite is considerably bleached, exhibiting a bronzed appearance. In 

 one specimen collected, the orthoclase, which is white, and sometimes 

 forms porphyritic crystals an inch long, does not show much evidence 

 of decomposition ; but where the section is exposed to the mechanical 

 action of the sea, the felspar has been, as a rule, completely removed. 

 In addition to the white orthoclase, there is another, and not so 

 abundant, felspar, which, when present, appears to form a zone on 

 the extreme edges of the granite-bands, in place of the white felspar. 

 In colour it is pink, with a tinge of orange which becomes more 

 pronounced as decomposition advances. Porphyritic crystals of this 

 felspar were not seen. 



When traced into the greisen the felspars gradually disappear. 

 Close to the granite, and, in the case of the porphyritic crystals, 

 nearer the quartz-veins, their form can still be distinguished in the 

 mass of quartz and mica ; but more generally they are completely 

 lost. A few of the biotite-crystals can also be seen in the greisen, 

 but they are here even paler than in the granite. 



The original quartz-grains, which, both in the granite and in the 



