144 EEV. E. HILL AKD PKOF. T. G. BONNEY ON THE 



mineral can be recognized in the Saignie Bay rock, but the 

 alteration of the rest of the matrix makes it difficult to say whether 

 there was also a glassy base, or whether the whole was rather 

 minutely crystalline. Probably it belonged to the kersantite division. 



So far as can be ascertained, the matrix of the Gouliot caves 

 dyke, if not minutely holocrystalhne, has been crowded with lath- 

 like crystallites of felspar, exhibiting sometimes a slightly tufted 

 arrangement, so that this also is a member of the same group. In 

 both these rather larger crystals of felspar have probably been fairly 

 common, but they are now too decomposed to admit of more than 

 conjectural recognition. 



The Havre Gosselin rock contains the largest and best-defined 

 representatives of the supposed pyroxenic constituent, and appears, 

 when viewed by ordinary light, to have a glassy base, the pisolitic 

 structure being indicated by rather irregular-shaped patches of clear 

 brown glass, separated by similar material of a sap-green colour, 

 Neither exhibits a trace of radial structure. The former, generally 

 at the edge, sometimes here and there within, becomes slightly 

 granular in structure, as if the colouring matter had begun to 

 aggregate, thus clearing the glass ; the green part is more uni- 

 form. In both are trichites and granules of opacite, with rather 

 numerous thin belonites. Felspar microliths appear rare, but in 

 one of the streaky portions mentioned above, in which also the 

 colouring matter has begun to separate out, they are fairly common 

 though very minute. At the same time, when the crossed nicols 

 are used, indications of an extremely minute devitrification are 

 perceptible, at any rate in all the brown parts of the slide ; still, 

 that it has been once a glass seems indubitable. A porphyritic 

 structure is visible to the eye in a specimen collected by Mr. Hill 

 from another part of this dyke, small white crystals about one- 

 tenth of an inch long being scattered about. But the microscope 

 shows them to be rather larger examples of the supposed pyroxenic 

 constituent. In this specimen the base (which in part is still a 

 true glass) is nearly all brown in colour, being rarely green and 

 giving a very faint indication of the globular structure in only one 

 corner of the slide. Probably this rock belongs to the same general 

 group as the others.^ 



The last described dyke, by its mode of occurrence, indicates 

 that in all probability these mica-traps are later in date than any 

 other igneous rock in the island. In Britanny and in Cornwall 

 and Devon, the mica-traps are found cutting rocks as late as Car- 

 boniferous, but are probably pre-Mesozoic in age. The mica-traps 

 of the Channel Islands most likely belong to one and the same 

 series of disturbances as these." The more the geology of the 

 region is studied, the more close appears to be the connexion of 

 those two districts with that group of islands. 



^ The group of the mica-traps in the matter of nomenclature seems to 

 suffer from excess here and detect there, not much less than when Prof. Bonney 

 wrote in 1879. 



^ Generally they cut crystalline rocks, but one in Alderney is intrusive in 

 gres feldsyathique (Hill, Quart. Jouru. Greol. Soc. vol. xlv. (1889) p. 384). 



