observed on the Rocks on the Shores of Greece. 835 

 zone is delineated in a manner still more evident. It is a groove 

 more or less deep, and in which the rock is naked ; its height is 

 not so uniform as in the preceding case, because it is often 

 formed by the reunion of little caverns or cavities, and thus its 

 height is elevated or depressed with them ; yet seen at a certahi 

 disfance, we recognise a constant mean height, and that within 

 the limits which I have assigned to it, a certain indication of a 

 permanent phenomenon, and of determinate limits. 



The result of the breaking of the tides upon the shore pro- 

 duces, on the contrary, effects which are not well defined, are 

 without a determinate elevation, and, as we shall soon see, alto- 

 gether different from the former. 



(Fio-. 1. a). The lower part of this groove, of which I have 

 not ye't spoken but to demonstrate the existence of a Mediter- 

 ranean tide, is attached to a nearly horizontal ledge or table, 

 which prevails at some centimetres beneath the level of the sea, 

 extends some yards in breadth, and terminates abruptly by a 

 sudden increase of the depth of the waters. 



This ledge exists generally wherever the shore is rocky, yet 

 it is sometimes concealed by descending debris, and is scarcely 

 perceptible in very inclined shores, where it is confounded with 

 the submarine prolongation of these debris. 



On the other hand, in the localities where the rocks are easily 

 destructible, as when greensand and its clays bound the sea, this 

 submarine horizontal ledge extends very far forward from al- 

 most vertical beaches. I have observed it in the bay of Modon. 

 There it extends out from 50 to 200 and 800 yards from 

 the shore, slopes gradually, and then it sinks rapidly to a 

 great depth. At its surface the sea is often only some inches 

 in depth. The bottom, perforated by fuimels in which the tide 

 seems to produce the effect of a wimble, is again covered with 

 slimy soft argillo-calcareous mud, which often leaves bare the 

 vertical sections of the greensand strata ; thus it is not a slope 

 slightly inclined, formed by debris produced by the tide, but 

 the result of an action of long continuance. This table or ledge 

 forms a submarine bank on which the tide breaks, and of which 

 it weakens the force ; consequently, the destruction uniformly 

 tends to cease, and we are convinced that it has attained its 

 limits in many places. 



