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XX. 



ON THE STEUCTUEE AND OEIGIN OF THE QUAETZITE 

 EOCKS IN THE NEIGHBOUEHOOD OP DUBLIN. By 

 PEOF. W. J. SOLLAS, D.Sc, LL.D., F.E.S. (Plate XV.) 



[Read ^ gp.-nT)TT>T^Y 17^ ifiW. ,] ^- 



Tlie earliest reference to the Cambrian Quartzite of this district 

 that I have met with is by Dr. Fitton (1), who thus early (1811) 

 calls attention to the characteristic conical form presented by 

 quartzite mountains like the well-known " Sugar Loafs " of 

 Wicklow, and suggests that it is " in some measure characteristic 

 of mountains composed of this rook." 



Weaver, (2) in his excellent description of the South-east of 

 Ireland, gives a good lithological description of the quartzite. In 

 colour it is " white, usually light yellowish or grayish white, some- 

 times more or less deeply stained yellow, red, and brown. Its 

 exterior is often of a glazed brilliant white, as if a solution and 

 reconsolidation of the surface had taken place." We may here 

 observe that the last sentence apparently refers to slickensided 

 surfaces, though it is curious that so acute an observer does not also 

 mention the striae and grooves so invariably associated with such 

 surfaces. Its " structure varies from the perfectly compact splintery 

 to the close-grained granular ; sometimes it contains small, well 

 defined, rounded grains of quartz, which are frequently of a 

 different colour from that of the base, even pale pink or purplish, 

 as for example in the peaks of the two Sugar Loafs and Bray 

 Head ; it contains, very rarely, rounded and angular grains of 

 felspar and a few scattered minute scales of mica. The pure clay 

 slate is yellowish. . . . These two rocks are interstratified, not only 

 on the large, but on the small scale," 



" All these rocks are more or less traversed by small contem- 

 poraneous veins of pure white quartz, which in their range fre- 

 quently follow the line of the dip. Quartz rock in particular is 



