54 PEOF. T. G. BOXKEY AND THE EEY. E. HILL : [Feb. 1 9 12,. 



but the granites and the cliorites, which are rather coarse-grained 

 rocks, run up to a junction with little, if any, change of structure y 

 and even the microgranitic dykes not only appear to have cooled 

 uniformly and have a close weld, but also in some cases have actu- 

 ally melted the adjacent rock, ns though it too, when they were 

 intruded, had been at a high temperature. Besides this the fact 

 that, wherever they may occur, they vary little in texture, indi- 

 cates that, although they cooled when the underground temperature 

 was too low for them to crystallize as ordinary granite, it was 

 fairly uniform in all parts of the islands. But the small group of 

 acid dykes, mentioned above, seems to have been injected into com- 

 paratively cool rocks, which is another reason for regarding them 

 as rather late in date. 



The granite is intrusive in sediments of Brioverian age (Schistes- 

 de St. Lo) in Jerse}" as well as in Xormandy, as, for instance, at 

 Avranches in the south-west and Yire in the south-east. In this- 

 province also granite-pebbles occur in a conglomerate beneath the 

 Gres armoricain.^ Thus it cannot well be earlier than the 

 close of the Archaean Era, and the microgranites must be rather 

 later ; while the diorites are more ancient, and the granite, now 

 represented by gneiss, oldest of all.- The relation of these to the 

 hornblende-schists of Sark and the Lizard must be reserved for a 

 subsequent discussion. 



That rocks very similar in chemical composition should differ 

 widely in geological age need not be an insuperable difficulty. The 

 ejections of basic magma in the South-West of England must have 

 continued, at intervals, from Archaean to early Permian times. In 

 Scotland similar discharges were abundant in Carboniferous (perhaps 

 also Permian) and Tertiary ages. Some of the Scottish granites are 

 ArchcTan, others Devonian, and others Tertiary. So we need not 

 be surprised if, in the Channel Islands, a diabase dyke is occasionally 

 cut by an acid one, though tlie bulk of them are distinctly younger 

 than the latter. 



The prevalence of soda-bearing felspars in the granites and 

 microgranites, as well as in the more basic intrusions, suggests that 

 all these rocks may be results of differentiation from a magma, the 

 separation of which began at a very early period — the most basie 

 product amoug the diorites being represented by the ' pici'itic ' rock 

 of Port Albert (Alderney) and that of Bon Pepos Bay (Guernsey) ; 

 the most acid by the tonalites of the Grand Havre district and of 

 Little Sark. The latter are related to the granites, and these to 

 the microgranites : the more ordinary diorite affording about four 

 varieties, and becoming on the whole more acid towards the north 



^ A. de Lappareut, ' Les Eocbes EruptiTes de I'lle de Jersey' Ann. Soc. Sci. 

 Bruxelles, vol. xvi, pt. 2 (1892), a verv valuable papei'. See also his ' Traite 

 de G-eologie' 5th ed. vol. iii (1906) p. 1746. A paper by Mr. J. A. Birds 

 (Greol. Mag. dec. 2, vol. v, 1878, p. 79) contains some acute observations, 

 though it has suffered from the misleading influence of Ansted. 



^ Their crystalline condition, as described above, suggests that the diorites, 

 granites, and microgranites were not separated by very long intervals of time. 



