County Dublin^ Past and Present. 



II 



quartz-sand, like those of modern beaches, with here and there 

 a little pebble, the whole being cemented by silica into the 

 quartzite that we now find. 



While examining these ver>^ ancient deposits, now cemented 

 together and contorted, we can picture to ourselves the rivers 

 bringing down sand and mud into the sea, which sifted out 

 the materials, as it does now, and spread them out in strata 

 on the quiet floor. There are no signs of the huge flood- waves 

 and whirling catastrophes which some of the earlier geologists 

 invoked to account for the vast masses of material deposited 

 during geological time ; we are dealing with rocks at least as 

 old as the oldest known traces of living things, and we find 

 ourselves, notwithstanding, studying conditions marv^ellously 

 like our own, and very far indeed from the time when the crust 

 of the earth began first to cool around a molten ball. 



But further examination of the quartzites shows that they do 

 not always lie regularly, as the}^ should, among the shales. On 

 Howth itself, where they often look so distinctly stratified, the 

 late Mr. John Kelly, ^ in a paper full of accurate observations, 

 held that they were originally quartz-rocks lying lower than 



Fig, 2. 



Bed of quartzite in slate, east of the ISTeedles, Howth, The shale has 

 become cleaved under earth-pressure, and has passed into slate ; but the 

 resisting quartzite has become folded and broken up. Further move- 

 ment would leave mere "ej-es" of quartzite surrounded by slate. 

 Dimensions of surface drawn, about 25 cm. square. 



the shales, and that they had been intruded into the latter in 

 a "semifluid or plastic" condition by "volcanic or other ex- 

 pansive power." Mr. G. H. Kinahan^ has also contended that 



1 "On the quartz rocks of Co. Wicklow," C. S. D. v. (1S53), pp. 240 and 



255. 



2 "Geology of Ireland," pp. 14 and 196. 



