584 Proceedings of the Royal Irish Academy. 



but they are quartzites disintegrated in situ, and out of which the 

 iron has been leached. In places in the same county there are fairly 

 pure, silicious, ferriferous sands, which, if washed and then consoli- 

 dated, would make a good silicious rock. On Achill Island, county 

 Mayo, there is a small tract of pure silicious sand. At Ballydonegan 

 Bay, county Cork, and Bonmahon, county Waterford, there are pure 

 silicious sands, but they are artificial, being crushed rock detritus due 

 to the mine stamps. At Kilmainham "Wood, county Meath, and other 

 places in the gravels, there are a few very pure silicious sands ; but, 

 in the main, silicious sands are very uncommon in the recent Irish 

 accumulations. It must, however, be allowed that in early times, 

 under circumstances of which we are not now cognisant, nearly 

 pure silicious rocks have accumulated. Take, for instance, the group 

 of rocks extending N. E. from Ramelton to the shore of Lough Swilly, 

 north of RathmuUen, and we find them to be in places almost purely 

 silicious. Then, again, many of the Coal Measure grits in Munster 

 and Leinster are highly silicious. So that in old times, for reasons 

 with which we are now unacquainted, silicious accumulations may 

 have been more common than they are now. 



Although I believe that my theory as to the origin of quartz-rock 

 is nearer the truth than any other that has been put forward, yet I 

 feel that in its entirety it is not quite satisfactory. Take, for instance, 

 the quartz pipe-rocks of Sutherland, Scotland ; they look very like 

 typical quartz-rock ; but if they are such, how did they accumulate ? 

 And if from hot springs, how did the worms, who have left their in- 

 numerable pipes, get into them ? Possibly they were heat-proof like 

 salamanders. 



In the county Donegal there are two distinct classes of quartzite 

 and quartz-rock. The older are, probably, of Pre-Cambrian age, the 

 newer may, possibly, be equivalents of the Ordovician ; but these two 

 quite distinct groups are left undistinguished in the maps and Memoirs 

 of the Geological Survey, so that their age is at present quite obscured. 

 The newer, belonging to the Kilmacrenans, seem partly to corroborate 

 my theory, but partly the reverse. Some of these arenaceous rocks, 

 as it appears to me, must have been sandstone, while associated with 

 them are rocks that I would call quartz-rock. In some places the 

 latter rocks have well-defined boundaries, and can easily be separated 

 from the others, but in places the quartz-rock seems to graduate into 

 the quartzite. Similarly, in the tracts of silicious rocks of Sligo, 

 Mayo, and Galway, in some places the rocks are decidedly quartz-rock, 

 in others quartzite, but in some places it is hard to say in which divi- 



