180 



Scientific Proceedings, Royal Dublin Society. 



the rock is composed, but with a careful adjustment of the sub-stage 

 condenser these are clearly distinguishable. That they are not seen at 

 once is owing to the absence of earthy impurities and to a reduction 

 in the quantity of sericite present between the grains, this mineral 

 occurring in only the thinnest possible films, and frequently being 

 entirely absent over certain areas in the slice. The quartz grains 

 frequently come into actual contact, and thus form a mosaic 

 similar in some respects to, but markedly different in others, from 

 the quartz mosaics of such igneous rocks as granite. Restricting 

 our attention, first, to those grains which are plainly margined by 

 sericite, one readily detects in them rounded outlines of the most 

 unmistakable character. Professor Blake has himself admitted 

 the presence of rounded pebbles in the quartz knobs, and though 

 he seeks to explain their formation in an exceptional manner, I 

 see no reason for regarding them as other than ordinary water- 

 worn grains ; their presence suggests a similar origin for the 

 remaining constituents of the rock. In the next place, it is to be 

 observed that most of the grains are clouded by a vast number 

 of liquid and vapour cavities, frequently collected together along 

 planes. These are far more numerous than in our Dublin 





Fig. 4. 



Fig. 5. 



Figs. 4 and 5. — -Sectioa of quartzite from near Amlycli, slio-wing secondary growth of 

 quartz on rounded grains. Fig. 5. — Somewhat more highly magnified (thestrise 

 on the left-hand border indicate sericite). 



quartzites, and give a very marked appearance to the section. 

 They are of importance to us in this study since they serve in 

 some instances to mark out the original outline of a grain that 

 would otherwise have been lost, and this outline is rounded. 

 By the fortunate preservation of the original outlines of the 



