Rock- Basins 'with the Spheroidal Structure of Granite, fyc. 34 1 



manifest, and since it is impossible, in those from which the 

 inferences had at first been drawn, to decide which of the two 

 causes of desquamation has operated, Dr. Macculloch's ori- 

 ginal deductions appear to be unshaken by this subsequent 

 discovery. 



It is to be observed that no facts have been adduced, which 

 prove the absence of the spheroidal structure, in those masses 

 of rock, whether granite or trap, which desquamate in a man- 

 ner evidently unconnected with structure, and depending only 

 on some unknown effect of the weather. In pursuing the 

 inquiry, therefore, it will be interesting to ascertain whether 

 rock-basins are formed in such masses of granite, or not. If, 

 however, the opinion expressed in this paper, that the phas- 

 nomena of rock-basins are connected with a concentric struc- 

 ture in the rocks on which they occur, be well founded, it will 

 afford the means of determining, whether, in those cuboidal or 

 prismatic blocks of granite, in which desquamation takes place 

 " all round the surface, respecting some imaginary point or 

 centre, and promising, in the progress of time, to reduce the 

 whole to a smaller and more spheroidal mass*," to which of 

 the two assignable causes the process must be referred. Thus, 

 since the granite of Cornwall and Devonshire is characterized 

 by this mode of decomposition, while it is also abundant in 

 rock-basins, we should infer, that the concentric desquamation 

 it undergoes, is really dependent on the internal concretionary 

 structure to which it had originally been referred. 



To conclude, for the present, these remarks on the geolo- 

 gical history of rock-basins : — Mr. De la Beche, in the expla- 

 nation of his " Sections and Views illustrative of Geological 

 Phenomena," lately published, after quoting Dr. Macculloch's 

 paper On the Granite Tors, as explaining the Views taken from 

 it of the Vixen Tor, the Cheesewring, and the Logan at Castle 

 Trereen, observes, " Looking at these drawings, and taking 

 into consideration the comparatively little waste which the 

 objects they represent now suffer, it would seem to require a 

 long lapse of time to produce the effects we here witness." 

 In this opinion every one must agree; but the present writer 

 would suggest, from a few observations of his own, that more 

 rapid, though far less extensive changes, are effected by that 

 particular species of action, which has produced, and is now 

 producing, as well as destroying, rock-basins ; and the effect 

 of which, in dividing masses of granite, and changing their 

 figure, is well exhibited in the Tors of Carnbrea. 



* Dr. Macculloch's paper last quoted, p. 239. 



The 



