Such contemporary horizontal platforms may be seen par excellence 

 in the Pacific and around the shores of Australia. These regions are 

 endowed with widespread limestone coasts, which, because of their 

 solubility in sea-water in the inter-tidal belt under certain highly special- 

 ized bio-chemical and physico-chemical conditions on the warmer reef- 

 fiats, are particularly favourable to the rapid erosion of these broad 

 horizontal platforms. It makes no difference whether the limestone in 

 question is of coral origin or is a massive chemical rock, or yet again is 

 the sandy calcareous eolianite commonly found on more arid coast-lines. 



In certain favoured places these contemporary platforms range up to 

 half a mile in width ; in others perhaps only a few feet. Normally they 

 are horizontal, though the outer part may be undercut and slope away ; 

 or it may have an elevated rim. The inner part may be truncated sharply 

 by cliffs ; or again it may rise in a gently sloping ramp, particularly so 

 in exposed places where there is a considerable swash. 



It should, however, be emphasized that horizontal reef platforms are 

 also found in rocks other than limestones, such as shale, sandstone, schist, 

 and so on, but massive rocks like granite and basalt are so resistant to 

 subaerial and intertidal weathering that, in the limited time available 

 since the last eustatic change of sea-level (possibly less than one 

 thousand years), no appreciable benching has occurred in them. There 

 are thus ideal bench-forming rock types, and others which are not at all 

 favourable. 



The explanation of this horizontal benching has been sought with 

 considerable curiosity. Investigations are still in progress. But this 

 much at any rate is clear : as Bartrum, in New Zealand, has already 

 very satisfactorily demonstrated, subaerial erosion operates rapidly 

 and deeply on many rock-tj^pes, but is arrested in depth by sea-level, 

 for alkaline sea-water acts as a preservative for most rock materials, 

 in contrast to the destructive character of acidic rain-water. Thus, 

 below the inter-tidal belt, the erosive action of the sea is negligible in 

 contrast to that of the atmosphere ; the critical zone of erosion is in 

 the inter-tidal levels, where rock materials, loosened and partially 

 disintegrated by subaerial forces, are etched, abraded, quarried, and 

 washed away by the surf. 



In this same critical surf zone certain curious temporary chemical 

 changes take place. Although sea-water is normally alkaline, here the 

 action of the surf causes large amounts of CO.-, to he taken into solution, 

 thus lowering the pH and rendering the water and spray temporarily 

 " acid " and thus corrosive. Drop in temperature in the water on the 

 reef flat at night and, in the absence then of photosynthetic removal of 

 CO 2 by plants, normal respiration of plants and animals in the inter- 

 tidal area will all tend to the same end — that is, an increase in acidity. 

 The relative importance of each factor is yet to be worked out.^ 



(1) I have recently come across a reference which helps to explain the 

 unfavourable reaction of certain rocks to bench-forming processes. Half a century 

 ago J. Joly, of Dublin, carried out careful experiments which demonstrated that 

 aerated sea-water dissolved various silicate minerals and igneous rocks much faster 

 than did fresh water similarly aerated. Hornblende is eight times ; orthoclase 

 fourteen times ; obsidian four times ; and basalt three times more soluble in salt 

 than fresh water. (Joly, J. (1901) : " Experiences sur la denudation par dissolution 

 ■dans I'eau douce et dans I'eau de mar." C.R. Congr. Geol. Int. (VHI, Paris, 1900)., 

 Vol. 2, pp. 774-784.) Naturally there is greatest aeration (acidity) of the sea- 

 water in the intertidal zone, so that one would naturally expect some sort of bench 

 to form, but the solution also goes on below this belt, so that one would not look 

 ior quite the same sharp outlines found on the ordinary benches. 



3^9 



