Rock-Basins with the Spheroidal Structure of Granite, $c. 339 



the direction of the axes of the spheroidal structure and any 

 of the surfaces of the beds composing the masses of granite? 

 Dr. Macculloch observes, (Classification of Rocks, p. 227,) 

 that "the great lamina?, or beds of granite, are often vertical, 

 as well as horizontal or inclined ; and it thus presents con- 

 tinuous smooth precipices laterally, while, above, it terminates 

 in sharp peaks." Now the beds of granite in Cornwall and 

 Devon are horizontal, or inclined ; and we find the rock- 

 basins on their upper surfaces, which are parallel to the direc- 

 tion of the beds : — are the granite beds of Scilly, presenting 

 basins on their perpendicular faces, placed in a vertical posi- 

 tion* ? 



If we view these rocks on the grand scale, and consider the 

 schorl-rock merely as a local modification of granite, which, 

 in a strictly geological sense, is undoubtedly the truth, we 

 may infer that the appearances at Roach, — which have been 

 regarded, in a former paragraph, as indicating the absence of 

 the spheroidal structure in that mass of rock, — are merely 

 the consequences of the vertical position of its constituent beds; 

 and the general aspect of these rocks gives indeed some weight 

 to this supposition, agreeing as it does with the character, de- 

 scribed, in the foregoing quotation, as resulting from that dis- 

 position of the beds. But in this case, the conjecture that 

 the form of the rock-basins in true granite is connected with 

 the spheroidal structure of that rock, will perhaps derive still 

 further support, from the difference of form in the rock-basins 

 at Roach, which must, under this view, be regarded as ex- 

 cavated in the summits of the vertical laminae ; while the 

 channels have been worn in their edges and sides. Still 

 further would it seem, from this view, that the position and 

 nature of the rock-basins, is connected, in some manner, with 

 the direction of the axes of the spheroidal structure, existing 

 in the rock. 



In relation to this part of the subject, it may be well to 

 make a remark, on the occurrence of rock-basins in the mill- 

 stone grit of Ashover, mentioned near the beginning of this 

 paper. If the conjecture, that the form, and the topical situa- 

 tion, of these cavities, are connected with the internal struc- 

 ture of the rocks on which they occur, be correct, some other 

 indications of this structure, either in the obvious external con- 

 figuration of the rock, or in the mode of its decomposition, may 



* One obvious reason why rock-basins should be more frequent on hori- 

 zontal than on vertical or highly-inclined surfaces, is the greater aptitude 

 of horizontal surfaces to retain water ; but the form and disposition of 

 the basins seem to indicate a further and structural cause for this fact, 



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