KiNAHAN — Quartz, Quartz-Rock, and Qiiartzite. 583 



If quartz-rock is only silicified ordinary sandstone, how is it that 

 one class of sandstone has been silicified, while all the other associated 

 t^andstones are not so? As in the following places: — In the county 

 Wexford, viz. at Eoney's B,ock on the east coast; in the Forth Moun- 

 tains, from Wexford westward to Taghmon, and thence southward to 

 the sea coast of Bannow ; in county Wicklow, in Carrick Mountain, 

 and from that S. W. to Macreddin ; also in the Greystones and Bray 

 Head, etc., districts ; and in county Dublin, in Howth and Ireland's 

 Eye. In these several districts the associated rocks into which the 

 quartz-rocks protrude, or intrude, are now, or were, before being 

 metamorphosed, series of grits, sandstones, and shales. How is it that 

 the quartz-rocks were silicified, while the associated grits and sand- 

 stone, often in juxtaposition, are not so? In some of these districts, 

 especially the Forth Mountain, there is a conspicuous difference be- 

 tween the quartz-rock and the associated metamorphosed grits and 

 sandstones. 



As pointed out in previous publications of the present writer's, all 

 silicious, calcareous, ferriferous, etc., springs, while in gentle ebulli- 

 tion, deposit the minerals in solution, these to be afterwards partly 

 broken up, whirled round and round in the orifice, and thereby 

 rounded, while, subsequently, when the springs are in violent action, 

 they are ejected and deposited around, where they are cemented into a 

 mass. Necessarily, a clastic rock thus formed is almost entirely 

 made up of the particles of the normal minerals of the spring due to 

 deposition. This, however, may not be always so, because if the 

 spring is deep-seated it may bring up pieces from one of the beds 

 through which it passes. Thus, the large calcareous spring on the 

 east margin of Lough Eea, county Galway, is continually bringing up 

 fine silicious sand. As far as my experience goes clastic quartz-rocks 

 are almost entirely made up of silicious fragments in a silicious, often 

 ferriferous, magma ; decidedly foreign particles only occurring at 

 certain centres, as if they were deposited in the immediate vicinity of 

 the springs. A spring origin for quartz-rock would, necessarily, in- 

 duce "false bedding," as each overfiow from the orifice would deposit 

 a separate layer. 



All ordinary arenaceous accumulations have, in general, a con- 

 siderable mixture, whether they be sands or solid rock. Under 

 pecidiar circumstances an accumulation may have been silicious or 

 nearly so, as at the present time we have such sands imported from 

 America. But in Ireland a pure silicious sand is rare. At Muckish 

 and in the Lough Salt range, county Donegal, we have such sands, 



