134 Transactions. 



The origin of sucli platforms is to be explained in great part by the 

 well-known fact that sheets of water protect rocks beneath them from 

 active chemical decomposition. 



Surface waters charged with atmospheric gases cannot percolate 

 effectively below sea-level, which must be a level of permanent water- 

 saturation ; the rocks below sea-level are thus relatively strong as compared 

 with those above, which have been subjected to subaerial decay, and will 

 resist weak wave-action, which is, however, competent to remove the upper 

 weathered layer. 



This fact, apparently, was recognized by Dana, for he says, " The 

 existence of this platform is owing to the protection of the sea from 

 wear and decomposition. Above, the material has disintegrated and been 

 washed away by the action of streamlets and the waves ; but beneath 

 the water these efiects do not take place."* 



But this action alone will not explain all the facts, or so it appears to 

 the wT:iter. He has noted, for example, the occurrence of unweathered 

 rock at a small depth below the surface within a very short distance 

 of well-developed platforms. He would suggest, therefore, that subaerial 

 weathering at the shore-line is progressive, and that, as wave transport 

 removes loosened spoil, fresh impetus is given to weathering, the water- 

 table at the new shore-line is lowered to the level of high water, and the 

 zone of decomposition retrogresses clifiwards. The gist of the hypothesis, 

 then, is that the platforms are due not so much to wave-attack upon 

 a definite zone of weathered rock as to the destruction of the clifi- 

 faces by subaerial erosion and the removal of the resulting waste by weak 

 wave-action. It is evident that the carving must occupy considerable 

 time, and if the wave-action is vigorous the normal shore profile may 

 be established and the sea-cliffs notched af their base in the well-known 

 manner wherever the character of the rocks is favourable. 



The high-water platforms of Auckland and North Auckland are invari- 

 ably surmounted by chffs of moderate height, a fact that is a necessary 

 corollary of the hypothesis of origin. The districts in question are areas 

 of drowned, maturely sculptured topography, with a climate favouring 

 rapid rock-decay, so that the zone of weathering must have reached a 

 considerable depth before depression gave rise to the present embayed coast- 

 lines. So long as wave-attack is directed wholly against the soft material 

 situated in the zone of weathering of the interrupted earlier cycle, the 

 normal shore profile obviously will result, and not before the rock of this 

 original zone has been removed at the level of wave-attack can the high- 

 tide platforms be initiated. At this stage cliffs with their height equivalent 

 to the depth of weathering will front the sea, and, after this, as they 

 retreat they will increase in height little by little towards the maximum 

 when the slopes of the spurs in which they are cut become reversed. The 

 section diagram illustrates these stages : A ^presents the sea-cliff at the 

 initiation of the platforms and B is a similar cliff at a much later stage, 

 whilst the dotted line indicates the hypothetical limit of the inwards- 

 advancing zone of decomposition at this stage. 



In conclusion, the writer wishes to thank Dr. C. A. Cotton, of Victoria 

 Uiuversity College, for obtaining references to literature inaccessible in 

 Auckland, and for friendly criticism. 



*J. D. Dana, Unit. States Explor. Exped. 1838-42, vol. 10, Geology, p. 442, 1849. 



