172 Scientific Proceedings, Royal Dublin Society. 



rock. In the country north of Aughrim and eastward to the 

 valley of the Avonmore are masses of quartzite, evidently meta- 

 morphosed quartz rock the age of which ... is uncertain, but 

 probably they are in the Cambrians, North-west of Wicklow 

 town, especially in Carrick Mountain, are long, more or less con- 

 tinuous cakes of quartz rock ; near Kilcoole, and at Bray Head, and 

 in the country to the west thereof, are dykes and protrusions of 

 quartz rock. One of the principal quartz rock dykes in Bray 

 Head runs south-west from the Brandy Hole, and the junction 

 between it and the other rocks is well exposed at the path above 

 the railway. It breaks up through the Cambrian rocks, and those 

 to the north of it seem to be altered, being very much mineralized, 

 still not so much as to destroy the fossils, this being a locality for 

 Oldhamia. Of the protrusions the largest and most remarkable 

 constitute the Oreat and Little Sugar Loafs. Another remarkable 

 hill of quartz rock is that called Shankill or CarrickgoUogan to the 

 north-west of Bray." 



Mr. G-erard Kinahan (7), although offering no opinion as to 

 the origin of the quartzite rocks, provides a more satisfactory 

 explanation of their apparently discordant position amongst the 

 surrounding slates : this will be readily understood by reference to 

 the map which he gives of the country about Bray Head, in which 

 the beds of quartzite are represented as broken up into a vast 

 number of dislocated blocks by repeated faulting. It is interesting 

 to compare the map given by this author of the bed of quartzite 

 extending inland from Brandy Hole with that of Mr. Kelly. 



In 1887, at the meeting of the British Association at Man- 

 chester, the author (8) stated that after examining the quartzite 

 with the help of the microscope he was convinced that it was a 

 slightly altered quartz grit, and he explained its deceptively 

 intrusive character as resulting from differential movements 

 during the folding of the country under pressure. Professor J. 

 F. Blake (9) suggested, in 1888, a different view in explanation 

 of certain masses of quartzite in Anglesey termed by him " quartz 

 knobs." I quote the following : — " They . . . occur as isolated 

 knobs of greater or less size, surrounded on all sides by shale or 

 schists, in whose orientation they take no part, and produce no 

 interference. They have in fact no orientation either on the large 

 or on the microscopic scale, and they show no signs of contortion. 



