Maw — On Suhaerial and Marine Denudation. 449 



exposed, must always be less than against a headland; a headland 

 is more or less assailable from three quarters, whilst an inlet can 

 only receive the first force of the direct waves. Waves rmining 

 along the shore can take little effect upon its rec3sses ; in short, the 

 projections of a coast, be they from whatever cause, must always be 

 more open to assault than its indentations, and in this a compensating 

 equivalent exists for any exceptional tendency towards an irregular 

 outline. With respect to the assumed erosive action of currents, 

 even if any large proportion played directly against the coast, they 

 would be unable to make a deep inlet, for motion cannot take place 

 up a ctd de sac — a "cushion of still water" would fill the recess, 

 deflecting the current at its movith, and thus neutralize its excavating 

 power ; furthermore, all the persistent currents of the ocean are on a 

 scale altogether disproportionate to the details of coast outline, and 

 for the most part take grand sweeps parallel with the coasts. 



On the Levelling Action of the Sea.- — If we examine the sea bed 

 between high and low water mark, on any cliff-girt shore, it is im- 

 possible not to be struck with the singularly level disposition of the 

 reef surfaces extending seaward, which once formed the foundations 

 of the old cliffs. Their general height will be a trifle above that of 

 low tide, and any irregularities of surface will not exceed one or two 

 feet. This well-marked lower limit to the erosive action of the sea 

 is not confined to hard rocky coasts, but will be found to hold good 

 in the softest strata. The fragile Wealden Sandstones to the east of 

 Hastings stretch out to seaward for many hundred feet as level reefs 

 just appearing above low water; and even the soft London-clay 

 spreads out on the Suffolk coast as a level plateau exposed at low 

 tide. There can be no stronger evidence of the impotence of the 

 sea to do much in the way of erosion below the tidal range; and if 

 a uniform level of land is maintained, it seems practically impossible 

 that anything but a level surface can be the result. In the case of a 

 coast being emerged or submerged, the progressive action of the 

 tidal range would, of course, produce a steep or gentle inclination of 

 surface according to the relative rates of erosion and change of level ; 

 but such inequalities must tend to be parallel with the shore (as 

 indicated in Pig. 8), and similar variations of surface, trending in an 

 opposite direction, could only be produced by sea erosion from local 

 differences in the oscillation of level ; but these, we know, do not 

 take place within the narrow limits of hill and valley undulations. 

 Furthermore, although you might get a single incline jjarallel to the 

 sea by its action, it is obvious this could not be repeated ia reverse 

 so as to produce an undidation, and still less possible would it be for 

 the sea to produce that intricate series of undulations skirting river 

 valleys opposed in their direction to the eroding sea line. It may 

 be perfectly true that marine terraces occasionally follow the 

 sinuosities of the land contours, but these are evidently the effect 

 of sea erosion superadded on previous contours produced by sub- 

 aerial denudation. 



On the Formation of Plains. — Li Mr. Mackintosh's paragraph, 

 entitled "Valleys Excavated by Streams" (p. 389), he appears to 



VOL, III. — NO, XXVIII. 29 



