1877.] of the rocks of the Channel Islands. 81 



my observation of these rocks goes to shew that the granitic struc- 

 ture is due to a metamorphism going on at moderate depths and 

 at temperatures far below the fusing point of the rocks. This 

 is quite a different thing from the assertion that some syenites 

 may not have been in a state of fusion at one time. This may 

 well have been, but if so, the granitic structure was not left on 

 their solidification but has been produced by subsequent meta- 

 morphosis. 



The granitic veins in the islands throw some light on this. 

 There are two kinds of granitic veins in Guernsey. First in the 

 cliff under Fort George the gneiss is traversed by many branching 

 veins. Some of these are ordinary quartz veins, but one at least 

 is granitic or syenitic. It is a small vein very sharply defined, only 

 about three inches thick, and the material filling it is a crystalline 

 mixture of quartz, felspar, hornblende and mica. There is not the 

 slightest reason to suppose it intrusive, in fact it is just like a 

 common quartz vein, except as to material, and I have no doubt 

 that it has been produced in a similar way. That is to say, a 

 crack in the rock has been filled up with material derived from 

 the rock itself, though in the case of this and other such granitic 

 veins I think the material must have subsequently assumed the 

 granitic crystallization. There are similar ramifying granitic veins 

 in grey syenite at Croc du Hurt^, the southern extremity of the 

 Greve d'Azette near St Helier's, Jersey. In neither place is there 

 any larger mass of the same structure with which they can be 

 connected, although there are greenstone dykes close by. I have 

 generally observed that the outside of a vein of this kind is 

 composed almost entirely of felspar with the ends of the crystals 

 pointing inwards. This probably has more to do with the process 

 (of metamorphism) by which the crystallization was produced than 

 with the mode in which the material got into the crack which it 

 has filled up. 



There is however another class of granitic veins, of which I found 

 an admirable example in Guernsey, in a quarry in Delancy Hill 

 adjoining the Grande Maison Road, near St Sampson's : in the 

 same hill in which the stratified quartzite occurs, but on the 

 opposite side of it, 300 or 400 yards off. The rock quarried here 

 is a highly crystalline hornblende rock with but little felspar in 

 its composition, but with various thin beds in it of a felsitic cha- 

 racter, parallel to one another and to the general direction of the 

 strata. These beds are some of them 12 inches thick, and others 

 not more than ^ inch, and in some places they are connected 

 by thin cross veins which were probably joints which have been 

 filled up. The material of these beds is in some places almost 

 a fine grained quartzite, and in other places, though felspar pre- 

 dominates in its composition, it still contains a notable proportion 



