194 T. Mellard Recide — Erosive Action of Sand-blasts. 



three nails in preserving the wood in the rear or lee of the nails, the 

 course of the sand-blast having been from left to right. These are 

 vpire nails that fastened the board horizontally to the upright posts. 

 The heads of these nails mark the original sawn surface of the board, 

 and indicate well the amount of the general denudation the board 

 has undergone. In the year 1875 I contributed a short article to 

 this Magazine on " Wind Denudation," describing little ridges of 

 sand on the shore left on the leeside of fragments of shells, or 

 sometimes whole shells, which have protected the sand from the 

 general denudation which always takes place in the upper moist 

 part of the shore during a strong breeze. These little ridges 

 I ventured to call ' eolites.' The wood ridges left in the rear of 

 the nail heads are the counterparts of these eolites, but they are 

 actually carved out of the board by the mechanical battering of the 

 sand-grains, whereas the eolites are due to the wind first drying 

 the surface of the sand and then blowing the grains away, except 

 where protected by the shell fragments. 



Another interesting feature is the rope-like appearance caused by 

 the truncation of bundles of fibres, shown by the minute transverse 

 markings on the photograph. This happens only where the grain is 

 not parallel to the surface of the wood. 



The effect on the oak staves is equally characteristic. Here, in 

 consequence of the regularity and parallelism of the grain, grooves 

 have been cut by the sand with the precision of a planing machine 

 (Fig. 2). 



Since the preceding was written another pine board has been 

 brought to me. It measures 2 ft. 6 in. x 6f in. The timber is 

 rather harder than in the one already described, and the grain more 

 regular. The sand has cut grooves of segmental section from three- 

 sixteenths to half an inch wide, deeply undercut on one side, the 

 ridges between the grooves being like a knife edge. There are 

 hardly any of the transverse markings to be seen, the grain being 

 parallel to the surface, and the whole has a smooth polished surface 

 to the touch. 



It is interesting to find that the continual attrition of these 

 quartzose sand-grains, many of them much rounded, in time cuts 

 deeply into the wood and develops the structure by differential 

 action on the harder and softer parts it operates upon, and also 

 polishes the surface. The time the wood has been exposed to the 

 blast is about seven years. What the velocity of the grains was in 

 a high wind I have no means of judging, but no doubt the air 

 currents were intensified in this wind-gap, and it must not be taken 

 as representative of the whole shore. 



EXPLANATION OF PLATE X. 



Fig. 1. — Portion of a pine board, 1ft. lin. x 5 in., eroded by sand-blast of the shore. 

 Fig. 2. — Portion of a split-oak stave, 5| in. x 2| in., ,, j, ,, 



