170 Scientific Proceedings, Royal Dublin Society. 



scarcely ever free from them, and they are sometimes almost in- 

 numerahle." 



" Q-uartz rock when in mass is unstratified."^ 



" The quartz rock of Shankill on the north side of the Dargle 

 is immediately connected with the granite, and judging from the 

 structure of this granite it probably passes into it ; but on the 

 eastern side of Shankill the quartz rock appears to be associated 

 with clay slate, the shingle of the latter being observable in the 

 fence which ascends the hill on that side." 



" In the great quarry at Howth are found iron pyrites, copper 

 pyrites, arsenical pyrites, and galena, gray ore of manganese, 

 brown iron-stone, and earthy black cobalt ore on the S. W. side." 



In a short account of the local geology drawn up for the visit 

 of the British Association in 1835, we find the distinction of the 

 quartzite into two kinds previously hinted at by Weaver, more 

 expressly stated in the following : — " The quartz rock exists in two 

 states, either alternating with the schist, and in that case decidedly 

 stratified, or destitute of all foreign intermixture, and in these 

 examples the stratification is very indistinct. The chief masses of 

 quartz are at Bray Head and Howth, in which it alternates with 

 schistose strata : Shankill and the greater and lesser Sugar Loafs, in 

 which no schistose strata occur." 



That the quartzite does not always behave exactly as might be 

 expected from a stratified rock appears to have repeatedly im- 

 pressed itself on geologists; but the most exact and searching 

 study of its irregularities we owe to Mr. John Kelly (4), who 

 demonstrated by numerous observations in the field that the beds 

 of quartzite, instead of maintaining an unbroken conformity with 

 the beds of slate with which they are associated, frequently cut 

 across them much after the fashion of intrusive igneous rocks. 



Mr. Kelly does not doubt that the quartzite was originally a 

 stratified rock, but he infers and rightly " that the quartz rocks 

 of the Sugar Loafs and Howth are not in their original position 

 with regard to the adjacent rocks." Mr. Kelly then proposes an 

 explanation in which he suggests that after its deposition the quartz 

 rock was rendered plastic by heat and then forced under great 



^ This is not quite correct ; obscure traces of bedding seldom fail, even in the most 

 massive quartzite. 



