128 G. H. HALLIGAN. 



wind over areas of any large extent. On a steep hillside 

 sloping to the north-east, the strong north-easterly winds 

 in summer will bend the growing timber towards the south- 

 west, and will cause all the well-known tree deformation 

 common on wind swept slopes. On the other side of the 

 hill, where the land slopes to the south, the winds from 

 that quarter will similarly affect the growth of timber on 

 the hill side which the north-easters do not touch. In this 

 case, the so-called dominant wind would be from the north- 

 east on one slope of the hill, and south-west on the other. 



As regards sand movement above high water, the move- 

 ment must, on this coast, as a general rule, be to the 

 northward, as the winds, having a velocity sufficient to 

 move sand in any quantity (say 20 miles per hour and over) 

 blow much more frequently from southerly than from 

 northerly directions, 1 the proportion being 1 to 1*78, during 

 a period of ten years. 



Whatever wind happens to be most effective over a cer- 

 tain area will heap the sand into dunes ; small obstructions 

 will form eddies, which prevent the ridges assuming a 

 regular form ; lengthy periods of ample rainfall will produce 

 luxuriant growth of reeds in parts where water accumulates, 

 and these in turn die and rot away to form humus, which 

 other and less hardy plants may grow in. It is nothing 

 unusual to find sand dunes in other parts of the world of 

 from 100 to 300 feet high, and although dunes of varying 

 heights will be found within a short distance of one another, 

 there is generally a noticeable uniformity in the height on 

 each beach. 



This is due to the uniformity in the duration of the par- 

 ticular wind which is mainly responsible for the dunes, 

 over the particular area, and should not, in the opinion of 

 the author, be taken to indicate earth movement unless 



1 " Sand Movement on the Coast of New South Wales," Ibid., p. 625. 



