— l62 — 



QUARTZYTKS AND QUARTZ-ROCKS. 



BY G. H. KINAHAN, M.R.I. A. 



Part I. — Quartz-Rock. 



There are quartz-rocks, quartzytes and quartzytes ; this 

 obscure statement being due to the present entanglement in 

 the use of the terms. The knowledge recently acquired by 

 the researches of the American geologists, both of the States 

 and the Dominion, demonstrates our ignorance of quartz3^tes, 

 and the importance of their being properly classified. But 

 before entering into this subject, quartz-rock only will be 

 treated of in the first part of the paper, especially the Irish 

 quartz -rocks. 



There are now tw^o geologists^ who would annihilate quartz- 

 rock from the list of Irish rocks ; but it seems to me that 

 they have published their views most unadvisably, as they ac- 

 knowledge that their whole experience in the Irish quartz-rocks 

 is derived from the small Dublin areas, while elsewhere they 

 could get convincing proof against their assertions. 



I first examined quartzyte and quartz-rock in Howth, Co. 

 Dublin; and quartz-rock in Bray Head about the year 1846; 

 but it was not till the yesLV 1870 that I wrote my first paper on 

 them, after I had studied them in Wexford, Clare, the west of 

 Ireland, etc. In the doctrines put forward by these more recent 

 obser\^ers, I do not see an}^ facts that invalidate my original 

 proposition. I find that Prof. Blake, independentl}^ came to 

 very similar conclusions to mine in regard to the origin of the 

 majority of the quartz-rock cakes and intrudes. 



Microscopists must not run away on theories in contra- 

 distinction to facts. If the dykes and cakes are due to springs, 

 the rock may be more or less fragmentary, let it be the adjunct 

 of a silicious, calcareous, or ferriferous spring. This can be 

 ocularly proved by observing the action of a spring. Usually 

 the welling up is more or less ^gentle and regular, and under 

 such circumstances the depositions are nearly homogeneous ; 

 but at times the welling up is most violent, and according to 

 the ratio of its force, it breaks off greater or less sized fragments 

 of the already deposited accumulation. These, as can be ocu- 

 larly proved, are whirled round and round in the vortex of the 

 vent, thus being more or less rounded prior to being ejected 

 from the spring to add to the accumulation. In such accumu- 

 lations, let them be silicious, calcareous, or ferriferous, the 

 inlying particles are of composition identical with that of their 



^ W. J. Sollas, " On the Structure and Origin of Quartzite Rock in the 

 neighbourhood of DubUn," Set. Proc. R.D.S., n. s., vol. vii., pp. 169-188; 

 G. A. J. Cole, " County Dublin Past and Present," Irish Nat., vol. i., 

 pp. 10-12. 



