Maio — On Subaerial and Marine Denudation. 



447 



cut a deep winding channel since the emergence of the land. At the 

 end of this main branch of the valley, a little below the waterfall, 

 the stream divides, each of the branches of which flow from a 

 short cul de sac, the main branch to the left over a boss of Porphyry 

 with a steep cliff-like front ; it is, however, not a true cliff produced 

 from below, but a very steep sloping mass resisting the cutting 

 power of the stream, the level of which above and below it repre- 

 sent the relative rate of degradations of the Porphyry and Silurian 

 rocks, the Porphyry appears to be metamorphic, and graduates into 

 the slates and shales, with which gradation of material the relative 

 steepness of the valley sides exactly correspond, the portion bounded 

 by the Porphyry being the steeper of the two ; the small branch of 

 the valley on the right hand side just misses the Porphyry and here 

 the cliff-like form is lost, and the little cul die sac- graduates upwards 

 to the watershed like the remainder of the valley. Now if the cliff- 

 like contour of the left hand branch had been produced from below 

 by the erosive action of the sea, would not the softer Silurian strata 

 have been the more easily assailed? and why has not the whole 

 valley that cliff-girt outline which we know the sea produces ? 

 There is nothing in the nature of the rock to prevent it standing in 

 the form of a cliff, for the immediate channel of the stream consists 

 of a gorge with vertical sides from 30 to 50 feet deep, the whole 

 valley (except the immediate river channel), although rather steep 

 in its conformation, has a graduated sloping contour, and there is no 

 reason why the sea should have singled out high u.p the valley a 

 little spot to leave its characteristic mark upon, whilst the more 

 salient parts were left untouched. 



^T^^^- " \ r?"^'-:/:^". ^''(^ii 



Fig. 7. — Pabt of the Coast of South America, sho"wing the want op 

 Correspondence between the Direction of the Coast-line and 

 THE Lines of equal height on the Land. 



The tendency of Marine Coast to be straight. — In my former paper 



