106 Mr. Moore's Remarks on the Origin of Rock-basins ; 



been left in the form of ridges, their edges only having been 

 rounded by the action of the elements. This fact is obviously 

 conclusive," &c. Not having seen these cavities, I shall not 

 venture to give any opinion decidedly respecting them. From 

 Mr. Brayley's description of them they appear to be very ir- 

 regular, and if he had not said that he did not find their sides 

 crumbly, I should be disposed to consider them as belonging 

 to that class of excavations which have been formed by natural 

 causes. If their origin were artificial, it is possible the ridges 

 might have been left through want of sufficient skill to re- 

 move them; or, the softer parts of the rock may perhaps have 

 been since worn away by the action of the weather, leaving the 

 porphyry projecting. But at any rate the reasoning from 

 these does not appear to me to apply to the rock-basins in 

 Devonshire, where, though the component parts of the granite 

 are said to differ greatly in durability, no projections occur. 

 From the former of these instances therefore no serious ob- 

 jection appears to arise to the artificial origin of the latter. 



Mr. Brayley, indeed, has himself felt a difficulty on this sub- 

 ject; for admitting the insufficiency of Dr.MacCulloch's theory, 

 he has proposed another objection to it, by saying (" Devon," 

 p. 292), " And indeed it would appear that some further 

 cause than the uniform texture of the granite must in reality 

 operate in the formation of these basins; for if that only were 

 the reason, the granite would be as much acted upon in a di- 

 rection perpendicular to its surface, as in those directions 

 which are parallel to it; and the depth of the basins ought 

 always to be equal to their diameters, or nearly so ; which, as 

 far as the writer's knowledge extends, is seldom, if ever, the 

 case. And the occurrence of the rock-basins on the vertical 

 faces of the granite at Scilly would seem to be a further cor- 

 roboration of this idea; for it is difficult to conceive how the 

 action of water could produce such cavities in this situation, 

 unless it were aided by the tendency of the rock to disintegrate 

 more easily in certain directions, with respect to the planes 

 of its surfaces, than *in others." The reasonableness of these 

 remarks is sufficiently obvious; and the further cause than 

 the uniform texture of the granite necessary to the pro- 

 duction of rock-basins, Mr. Brayley supposes to be found in 

 the spheroidal structure of this rock. His reasoning appears 

 to be thus: The constituent parts of granite are spherically 

 arranged ; in other words, a mass of granite consists of a num- 

 ber of spheres; and as disintegration on the flat surface of 

 the rock takes place, this kind of structure is favourable, in 

 some way or other, to the formation of these cavities : and 

 that the structure of granite is thus spheroidal in all cases, 



Mr. 



