340 The Irish Naturalist. 



CORRESPONDKNCE. 



Quartz, Quartz-rocks, and Quartzltes, 



To the Editors of the Irish NATURAiyiST. 

 Mr. Kinahan has been so good as to forward to me my letters 

 addressed to him on this subject, and I will ask you to favour me by 

 printing the whole of my second letter to him, except the two closing 

 paragraphs which deal with other and private matters. The exact 

 bearing of my study of the structures of siliceous sinters on the origin of 

 quartzites, etc., will then be rather more clearly seen, 



W. w. Watts, 



28, Jermyn-street, S.W., 



January loth, 1895. 

 Dkar Mr. Kinahan, 



I have had three examples of Icelandic sinters cut, and examined 

 them with a view of testing the relationship between them and quartzite. 

 Two out of the three show nothing but irregular layers of deposit which 

 undoubtedly give rise to the cauliflower-like surface of the sinter. 



This consists of opaline silica which has very little reaction on 

 polarized light and is consequently utterly difl"erent from the crystalline 

 quartz which makes up the bulk of quartzites. Were such a rock broken 

 up and recemented by geyser action it would present similar features. 

 The broken grains would consist of opaline silica cemented by similar 

 material, and would be different from any quartzites I know, which show 

 grains of quartz — like sand-grains — which have grown larger by the 

 addition of crystalline silica to their edges. 



The third slice, however, shows little nests or pockets full of minute 

 angular sand-grains, chiefly of quartz, but also chips of felspar and other 

 minerals. Here and there these little pockets are embedded amongst 

 the sinter, the rest of which has the character already described. These 

 grains, however, are embedded in, and cemented by opaline silica, 

 quite unlike that of quartzites, and there is no trace of any secondary 

 quartz. Nor are the grains in any way enlarged by the deposit of new 

 quartz at their edges, but these edges are either sharply splintered or 

 very slightly rounded ; in other words, they are of the form of 

 ordinary clastic sand-grains such as may be seen in almost any fine - 

 grained sandstone. 



Ido not say, of course, that some sinters may not have structures 

 like quartzites or quartz-rocks, but I have chosen those examples from 

 our collection — not a very large one — which looked to the naked eye most 

 like quartzites and, on slicing them, I find no character in them which 

 could possibly suggest that any of the quartzites or quartz-rocks which 

 I know liave been made by the same process. 



W. w. Watts. 



