to a height between high and low tides," though in this case Dana does 

 not mention subaerial action. It is not without significance that Dana 

 had travelled the Pacific extensively, including Australia and New 

 Zealand, while Gulliver, Davis, Barrell, Johnson, and others formulated 

 their ideas on the isostatically unstable, and therefore abnormal, coasts 

 of New England (which are suffering post-Glacial reactions.) 



Even Sir Archibald Geikie (1903, p. 575), while strongly emphasizing 

 the forces of mechanical marine erosion, was careful to point out that : 

 ^' Were it not for the potent influences of subaerial decay, the progress 

 of the sea would be comparatively feeble." In Norway, recognizing the 

 efficacy of subaerial erosion, Nansen (1905) believed that only such loosen- 

 ing and weakening could break down the extremely hard metamorphic 

 (Caledonide) rocks, enabling the waves to move the debris to form the 

 famous " Strand-flat." 



Actual observations by W. von Zahn (1909) on the rocky coasts of 

 Brittany and Normand}^ disclosed two distinct zones above and below 

 mean sea-level : a " Schliffzone " or " Smooth Zone " below, affected 

 b)y mechanical erosion ; and a " Spritzzone," or " Brandungskarrenzone " 

 (Spray-etched Zone), above, affected by chemical erosion. Kayser (1923) 

 pointed out in any case that all three processes of purely marine erosion, 

 b)ioIogical, physical, and chemical are restricted to the absolute upper 

 limit of marine erosion, and therefore tend to cut horizontally. 



The rather special position of limestone in relation to chemical solution, 

 as indicated above, has long been recognized, though exaggerated by 

 Murray, Agassiz, Gardiner, and Crossland. Rapid solution of limestone 

 in ordinary open sea-water certainly does not take place beneath the 

 surface. On the other hand, recent observations by Macfadyen (1930), 

 Kuenen (1933)," and Fairbridge (1948) show conclusively that chemical 

 erosion of limestone is accomplished by physico-chemical and/or bio- 

 chemical processes of sea-water in its surface few inches near the shore. 

 In this way broad, flat marine benches are eroded, the level of which is 

 at the low-tide limit. Maximum erosion, in protected places — i.e., 

 where the actual sea-level is not mechanically raised by wave action — • 

 is at mean sea-level, and cliffs are deeply undercut at this level. As 

 Kuenen says : " The solvent action is limited between the tidal range. 

 The action of the sea is that of ' sawing ' into the limestone." 



Apart from the exceptional position occupied by limestone in this 

 broad picture, the weakness of ordinary rocks to subaerial decay and their 

 resistance to submarine mechanical attack has been specially emphasized 

 by workers in New Zealand, where particularly fine examples of various 

 benches were exposed. Noted in ver\^ early times by Dana (1849, 1872, 

 1880), a thorough explanation appears to have been first suggested 

 by E. de C. Clarke in 1909 : " Rock-benches are developed in many places 

 along the steeper parts of the shore-line, more commonly in the 

 sedimentary rocks, but also in the volcanics. These benches consist 

 of shelves cut out of the solid rock, generally horizontal . . . 

 it seems possible to ascribe the formation of rock benches to the co- 

 operation of subaerial weathering, which causes the retreat of the 

 cliffs,' with marine transport, which removes the waste so-formed " 

 (in Bell and Clarke, 1909, p. 30). 



Bartrum has greatly developed this research into the New Zealand 

 benches, and while I fancy that he did not fully appreciate the 

 significance of eustatic changes, he clearly recognized the importance 

 of subaerial weathering above sea-level. While the lower part of the 



354 



