Vol. 69.] 



GEOLOGY OF -BARD SET ISLAND. 



517 



m 



k\ 



and they, as well as the map (PI. L), 

 should be referred to when the following- 

 description is read. It is not claimed, 

 however, that the folding indicated in 

 these sections is more than semi-dia- 

 grammatic, it being impossible to de- 

 termine in many cases -whether the 

 isoclinal folds (which are themselves 

 much broken) are anticlines or syucliues. 



(a) The Northern Coast. 

 (Fig. 1.) 



In the north-western corner of the 

 island, in the cliffs between Ogof Las 

 and Ban y Bhigol, the granite discovered 

 by Miss Raisin is exposed at five locali- 

 ties, but these exposures may represent 

 no more than a single sill repeated by 

 folding. Thus, when the two granite- 

 bands and the associated sediments on 

 the northern shore are examined, the 

 succession of the beds (fig. 1) is seen 

 to be such as it would be if they were 

 disposed in an isoclinal fold, whether 

 anticline or syncline. Between the 

 granite-bands occur thinly and evenly- 

 splitting, well-cleaved, green, sandy 

 slates, which are often white-crusted 

 and weather to a buff colour ; they 

 contain a central zone with quartzite- 

 bands that exhibit overfolding. Beyond 

 the granite-bands, in the same order 

 on each side, are cataclastic slates and 

 grits, followed by a slaty zone with 

 thick quartzite and limestone in large 

 lenticles and broken beds. The western 

 band of granite is a sill about 100 feet 

 thick, with a west-north-westward dip 

 at 60° to 80°, its lower edge following 

 closely, though with slight transgres- 

 sion, the cleavage-planes of the slates. 

 It is traversed by thrust-planes which 

 have driven it and the overlying beds 

 south-eastwards for short distances ; it 

 is fissile as though roughly cleaved, 

 and is so much crushed as to present 

 under the microscope the appearance of 

 a breccia. The eastern granite-band 

 is narrower, being about 40 feet thick, 

 but the difference in thickness offers 

 no special obstacle to the suggestion 



