586 Proceedings of the Royal Irish Academy. 



If quartz-rock was only an ordinary silicified sandstone, it ought,, 

 as a general rule, to contain more or less foreign particles. If, how- 

 ever, quartz-rocks are accumulated from springs, they ought to he 

 clastic, but the particles ought to be nearly all silicious, they being 

 derived from the breaking up of portions of the accumulation which 

 are subsequently re-cemented together. Such deposits ought also to 

 have false bedding, as each successive discharge from the springs 

 would be represented by a separate layer of accumulation. 



NOTES ADDED IN PEESS. 



A. — In the discussion it was stated that the silicious sinter deposited 

 from recent hot springs was quite distinct from quartz-rock, but an 

 examination of specimens from the hot spring deposits of New 

 Zealand shows that this is not so. My former colleague, Mr. "W. "W. 

 Watts, now of the English Branch of the Geol. Survey, has taken 

 the trouble to examine the specimens of the sinter from Iceland in the 

 Jermyn-street Museum. He finds the rocks are more or less clastic. 

 They show " irregular layers of deposition which, undoubtedly, give 

 rise to the cauliflower-like surface of the sinter." " The third slide 

 shows little nests or pockets full of minute angular sand-grain, chiefly 

 of quartz, but also chips of felspar and other minerals. "These 

 grains are embedded in opaline silica." 



B. — Quartz-rocks. — If this class of rocks is due to the out-pouring 

 from hot silicious springs, necessarily it is probable that some other 

 mineral or minerals may be present in solution. That this is nearly 

 invariably the case is proved by the minute particles of mica, &c., 

 developed in quartz-rocks by secondary action and by shearing ; also 

 by some quartz-rocks in places becoming felsitic. 



