308 Scientific Proceedings, Royal Dublin Society, 



but it is during the summer heat, when the water is shallow, that 

 the iron is deposited, partly by the ore being concentrated by 

 evaporation, and partly by the chemical agencies already mentioned, 

 which precipitate the soluble carbonate and sulphate, having more 

 power to operate. For these reasons we should have first plain 

 mud, then ferriferous muds with seams of ore, and then the same 

 alternations repeated. 



At times the iron-ore would be in excess, as whetlser the ore in 

 solution be either the carbonate or the sulphate, when the waters 

 are low and heated a quantity of the ferric oxide is deposited in 

 the shallow pools and along the beds of the rivers and their tribu- 

 taries, which stores are brought down by the first series of floods to 

 be deposited in the estuaries or such like still waters. 



The Ovoca river, county Wicklow, and such like rivers, which 

 receive mine drainage, carry down in suspension and in solution a 

 vast quantity of iron, to be afterwards deposited along the lower 

 parts of their courses and in their estuaries and outside in deep 

 water. Ferriferous river water also deposits iron in the lakes and 

 loughauns through which it passes. 



If, however, the loughauns and pools are shallow, the ore is 

 generally washed out of them by the autumnal and winter floods. 

 The stream that flows northward along the Maum valley, county 

 Gralway, gets quite thick and turbid during the summer heat, while 

 a quantity of the ferric oxide is deposited in the pools, to be washed 

 out of them and carried away by the subsequent floods. 



It is a remarkable fact in connexion with the ores in the allu- 

 vium of the Irish rivers, that these old deposits are often of con- 

 siderable thickness and extent, such not being the case with the 

 recent ones. Those thick deposits are now nearly always covered 

 with a greater or less thickness of alluvium. These peculiarities 

 may possibly be due to the leaching out of the colouring matter of 

 the rocks having been more active formerly ; that matter having 

 been removed to such a depth, the leaching now goes on very 

 slowly. 



Bog iron ores and ochres, although found in the alluvium of 

 lakes and rivers, are very often associated with peat or peaty accu- 

 mulations, and in general occupy hollows or nearly level surfaces ; 

 but not always, as some occur on hill slopes. When in hollows 

 and on flats the overlying bog may be due to the iron ore having 



