SoLLAs — On the Structure and Origin of Quartzite Rocks. 179 



some of tlie other quartzites, but occasionally one meets with a 

 microscopic quartz pebble wliicb, though it looks homogeneous 

 enough in ordinary light, yet reveals its composite character 

 between cross Nicols. All the other quartzites of the district, 

 those from Bray, as well as those of Loughereen, Howth, present 

 similar characters to the foregoing; all are obviously clastic rocks 

 which differed when first laid down as sediments in no distin- 

 guishable particulars from a typical quartz sandstone : subsequent 

 pressure, aided in some cases by percolating siliceous waters, is 



Fig. 3. — Section of red quartzite from Eed Eock, Howth. HigUy magnified. 



fully competent to account for the changes they have since under- 

 gone. 



How it is with regard to the so-called " quartz knobs " of 

 Anglesey one would hesitate, without detailed examination, to 

 decide, but fortunately I have in my possession a fragment of 

 one of these quartz knobs which Professor Blake pointed out to 

 me as a typical example of the kind. This, which comes from 

 near Amlych, I have had cut, and the thin slice prepared from it 

 shows, in my opinion, the characters of an altered sandstone. Its 

 general appearance differs somewhat from that of the quartzites 

 which occur in our vicinity ; at first sight one does not recognize 

 without the aid of polarized light, the individual grains of which 



SCIEN. PROC. R.D.S. VOL. VII. PART. III. Q 



