G. H. Kinahan — Re-arranged Glacial Drift. 115 



and in tlie co^ Limerick there are well-marked low cliffs that are 

 supposed to be Gontemporaneous. In south-east Ireland, near the 

 coast, gravels of this period seem not to be recorded.^ This, however, 

 may be due to the great encroachment of the sea on its soft cliffs, 

 which has obliterated all traces,^ except in the low places bordering 

 the lagoons, estuaries, and inland bays. The shelly estuary gravel, 

 on which a considerable portion of Dublin is built, seems to belong 

 to this period. 



Previous to^ mentioning the submerged bogs and forests that mark 

 the fourth period, it appears necessary to vindicate their real signifi- 

 cance, as it has been suggested that submerged and submarine bogs 

 do not prove a subsidence of the land, as they may accumulate in 

 lagoons or other places below the sea-level, and afterwards be sub- 

 merged by the sea breaking through the enclosing barrier, or the 

 bogs may become submarine by the sand bank being moved land- 

 wards. In some cases such a supposition may be correct ; but in 

 most cases (those of the Irish submerged and submarine bogs being 

 among the number) it could not be the case, as the bogs contain 

 roots, or '• corkers," m situ, of such trees as the oak and pine — timber 

 that would not grow except on drained ground ; besides, the trees are 

 of considerable size : consequently the land must have been above the 

 sea-level for a long time, while the oak and pine forest was growing 

 to maturity ;. while subsequently the land would seem to have begun 

 to sink, and the drainage to have become defective, causing the decay of 

 the forest,^ and afterwards marshes in which such trees as willows 

 grew, with peat producing plants succeeding. The latter state must 

 also have existed for a considerable time, when we notice the thick- 

 ;ness of the peat found in the submerged and submarine bogs, and 

 .know the length of time that the vegetable matter required to form 

 only a few inches of solid peat, took to grow before the peat had sunk 

 gradually below the level of the sea. Fresh water or brackish water 

 deposits must have accumulated in such places, which were succeeded 

 by marine deposits. Subsequently the land seems to have risen 

 slightly, as in most, if not all cases where these estuary lands have 

 been reclaimed, there is a greater or less thickness of the surface 

 portion, highly impregnated with iron, and in general separated 

 from the purer marl underneath by a stratum of shells. This highly 

 ferruginous portion seems to have been deposited in shallow water, 

 the iron being due to the mineral in solution having been deposited 

 from the water evaporated, while, when the water was deep, all such 

 solid matter would have been carried away. The oxide of iron being 

 deposited when the salts of iron, such as a sulphate or carbonate, 

 common in many waters, are decomposed by decayed vegetable or 

 animal matter, thus forming in the marginal or shallow portions of 

 lakes, estuaries, etc. The layer or stratum of shells separating the 

 two varieties of marl is evidently due to the change in the condition 

 of the water, as, when it became charged with iron, it destroyed the 

 animal life. Why this should have taken place suddenly is not 



^ Since the above was written, my colleague, Mr. E. T. Hardman, has drawn 

 :,attention to a beach of this period near Tramore, co. Waterford. 



