Kinahan — On Irish Arenaceous Rocks. 9 



colour." Imperfect pointing is most deleterious, as it exposes the 

 stones, as it were, iu flank, to freezing, solution by rain, hydra- 

 tion, &c. 



[In Ireland, although there are no very sudden changes hetween great heat and 

 cold, yet the frequent alternations during winter of freezing and thawing cause a con- 

 siderable amount of denudation, as can be seen in walls and natural rock-faces exposed 

 to the mid-day sun : the sudden changes from wet to dry in summer have also a marked 

 effect. "Wind, not only in this but also in other climates, has more power as a denu- 

 dant than it is generally given credit for. Strong winds in dry weather, carrying 

 gritty particles, will act similarly to " sand-blasts," especially in eddies round corners, 

 under-cutting the joints in buildings, and eroding grooves along the bedding and other 

 lines in natural surfaces. Wind after rain dries the saturated surfaces and enters the 

 minute shrinkage fissures, thus cutting out the particles, to carry them away and use 

 them as " sand blasts " elsewhere. If wind and rain are combined, the latter is driven 

 into the faces of walls, so that they have to be protected by weather-slating, cement, 

 or some such appliances. The effects of wind, combined either with carried particles of 

 sand or rain, may be observed in all exposed places — on buildings, monoliths, tombs, 

 surface-blocks, and cliffs, especially at the earth-line, where they are more or less 

 under-cut. In our climate some stones, such as granite and allied rocks, sandstone and 

 slate, seem nearly always to be protected from weathering by those lichens that grow 

 in sheets ; but those that grow in branching tufts from a small root seem to promote 

 decay. But all lichens seem to induce weathering in limestone, and even in slightly 

 calcareous rocks : the decay being apparently due to the acids generated by those 

 plants. 



In former publications I have shown that in all classes of rocks there are some that 

 are chemically hard, although physically soft ; while there are others which are physi- 

 cally hard, although they may be chemically soft. These characteristics must be taken 

 into consideration in selecting stones for different purposes. Chemically hard stones 

 may be most suitable for building purposes, while unsuitable for places where they are 

 subject to wear and tear ; while the opposite may be the case with physically hard 

 materials. 



In sea works, that are exposed when the tide is out, as also in those above high 

 water which are wetted during storms, or such like, some stones weather peculiarly ; 

 irregular holes wearing out; or, if tbey are cleaved rocks, the cleavage planes becoming 

 developed. Cement, which is an artificially-formed calcareous rock, is in some places 

 curiously licked out by sea or brackish water. This 1 have specially remarked at New 

 Ross, on the Barrow, Co. Wexford, where the walls were pinned with wooden pegs, 

 these being more durable than cement ; and also in a wall at the north end of the 

 Esplanade, Bray, Co. Wicklow. This action of sea-water seems to be in part the 

 mechanical result of the sudden wetting and drying of the rocks ; and in part the 

 chemical result of the action of acids, and the formation of salts that are easily 

 dissolved.] 



Paving-setts. — Paving must have been in vogue at a very 

 early date, as it is found in prehistoric structures, such as the large 

 crannog in Lough Rea, Co. Galway, where the stones were well 

 selected. In early times the stones principally used were the 



