Yol. 68.'] rETEOLOGlCAL NOTES 02^ GTJEENSEY, ETC. 53 



produced by tlie mixture of an acid with a more basic and perhaps^ 

 already solid material. The structure of these Guernsey dykes, 

 and the conversion into biotite of the hornblende in the diorite, 

 where it has been to some extent melted down, confirms the inter- 

 pretation already given of those banded ' granulites ' ; and indi- 

 cates that many of the Laurentian gneisses, though igneous in 

 origin, owe their distinctive structures to the fact that they cooled 

 in a different way from a normal granite.^ 



IX. Age of the Eruptive Hocks ik the Channel Islands. 

 [T. G. B.] 



Dykes of mica-trap, in most of the Channel Islands, break only 

 through gneissoid and igneous rocks, but in Alderney one cuts the 

 Gres feldspathique (Upper Cambrian), while in the Kade de 

 Erest and in the south-west of Britain " they are intrusive in Lower 

 ■Carboniferous strata. 



They all are probably of Permian or but slightly earlier ^ age. In 

 Alderney two greenstone dykes also cut the Gres feldspathique; 

 but in the other islands we can only say that these dykes (with two 

 exceptions) appear to be later than any other crystalline rock.* In 

 Cornwall and Devon the intrusive 'greenstones' are generally earlier 

 than the acid dykes (' buff elvans'), but the numerous basic dykes 

 at the Lizard, which must have been intruded when the peridotite 

 (serpentine) and the gabbro Avere comparatively cool, may imply 

 two ejections of similar magma at widely-separated intervals.' A 

 few acid dykes (the compact), such as those at Fort Clonque 

 (Alderney), Fort llegent (Jersey), and one or two at Castle Cornet 

 and three at Pleinmont (Guernsey), may be quite late, because the 

 first-named of these cuts not only a microgranitic but also a green- 

 stone dyke, and one of the last cuts a basic dyke. 



Still, as similar compact felstones occur as pebbles, rather abun- 

 dantly, at the base of the Gres feldspathique both in Alderney 

 and in the Omonville district, and as the rock has a considerable 

 resemblance to the acid lava-flows of Eastern Jersey, which are 

 now admitted to be below the Gres pourpre (basal Cambrian), 

 we should naturally suppose them to be of the same age. If so, 

 they are much older than the acid dykes of Cornwall. 



Another fact points to a rather marked gap between the ages of 

 these Channel-Island rocks. The diabase dykes, so far as we have 

 seen, are not coarsely crystalline, and frequently have chilled edges ° ; 



^ To put it briefly, the normal granite came to rest as a liquid, and then 

 crystallized : the gneissoid granite of ' Laurentian"' type crystallized as it was 

 coming to rest. 



2 J. J. H. Tealh ' British Petrography ' 1888, p. 350. 



^ Sir A. Geikie, ' Ancient Volcanoes of Great Britain ' vol. ii (1897) p. 96. 



* Except, of course, the mica-trap. 



5 This, of course, may indicate that our preliminai'y assumption is not 

 always valid. 



° This statement, although the number of instances is few, holds also of the 

 smaller group of acid dykes mentioned above. 



