174 Scientific Proceedings, Royal Dublin Society. 



They are not veins in form or in structure, but may have had a 

 similar origin. The only suggestion I can offer is that they are 

 the result of the cooling of hot water which has bubbled up and 

 eaten away the rock into a cavity, then deposited quartz on the 

 sides; in some cases has broken up again the first deposits and 

 rounded the fragments into pebbles, and has finally filled up the 

 cavity by the deposition of its quartz. In other words they are 

 the bases of pre- Cambrian geysers which may or may not have 

 succeeded in reaching the surface and erupting." The quartzites 

 of Howth and Bray are treated of with greater caution on page 

 535 of the same communication. Their general character how- 

 ever is, as Professor Blake informed me when examining them, 

 identical with that of the quartz knobs of Anglesey. 



Prof. Bonney (12) subsequently examined a thin slice from 

 one of the Anglesey quartz knobs ; his conclusion is that if it does 

 not represent a quartzite, he has never seen one. 



It is clear from the foregoing history that the division of 

 opinion which, from a very early period, manifested itself in 

 regard to the origin of the Leinster quartzites, has by no means 

 been removed by the newest investigations aided by the refine- 

 ments of microscopic investigation. In the following inquiry I 

 shall first attempt to show that the quartzites of this district are 

 of normal aqueous and clastic origin, and next to explain the 

 abnormal position in which they frequently occur amidst the 

 surrounding strata. 



1. Histological. Carrickgologan. — The presence of rounded 

 grains of quartz as much as a quarter of an inch in diameter has 

 been already recorded by the older observers ; in the course of half 

 an hour's ramble over Carrickgologan a good many such grains 

 may be observed ; it is frequently possible to extract them from 

 the parent rock, and they are then seen to resemble minute 

 pebbles. They certainly are water-worn, and suggest therefore a 

 sedimentary origin for the rest of the rock. This suggestion is in 

 every way confirmed by an examination of thin slices of the rock 

 under the microscope : even without the aid of polarized light one 

 can clearly distinguish the separate grains of quartz of which the 

 greater part of the rock is composed, their margins being well 

 defined by a thin layer of minute transparent colourless flakes of a 

 mineral which I take to be sericite, the refractive index of which is 



