Artificial Origin of Rock-Basins. 333 



a consequence of the tendency of the action which produced 

 them, to extend itself equally in every direction; which " the 

 uniform texture of the granite" (to which, merely, it is ascribed 

 by Dr. Macculloch,) would necessarily permit it to do*. Mr. 

 Moore also brings forward, as militating against the truth of 

 Dr. Macculloch's explanation, the facts that the rock-basins 

 " are found only on the tops of the tors, and sometimes on the 

 logan-stones :" and, resuming this argument, he inquires, " If 

 the basins were excavations produced by natural causes, why 

 are they not found on numerous other rocks and in different 

 situations ?" In reply to this it may be stated, that, in the 

 granite of the Scilly Islands, and in the millstone-grit of Ash- 

 over in Derbyshire, (which is associated with the coal-mea- 

 sures, and belongs, consequently, to a very different group of 

 rocks,) these basins do actually occur on the perpendicular 

 sides of the rocks. That they should be sometimes found on 

 the logan-stones, is also a consequence of their origin from 

 natural causes; since the same action of the elements, which, 

 operating on the angles and edges of the blocks of granite, 

 has produced the logans — which Mr. Moore had before cor- 

 rectly remarked " are clearly inartificial," — operating on their 

 exposed surfaces, has formed rock-basins. Hence, it w T ould 

 be remarkable indeed, if rock-basins were not sometimes found 

 upon logan-stones. 



An instance has already been cited of the occurrence of 

 rock-basins on other species of rock besides granite. It may 

 be mentioned, in addition, that there are deep cavities of this 

 description on the horizontal and on the slightly-inclined sur- 

 face of the magnificent mass of schorl-rock at Roach, in 

 Cornwall ; in many of which the present writer has found 

 grains of quartz and fragments of crystallized schorl, resulting 

 from the action that produced them; which is a circumstance 

 parallel to that related of the rock-basins in granite, by Dr. 

 Macculloch. It may also be useful to remark, in further ex- 

 planation of the process by which these cavities, in whatever 

 rocks they occur, appear to have been formed, that, on the 

 declivities of Roach rocks, where the water could not lodge, 

 it has worn deep channels, instead of producing basins. 



* The tendency of the action which has produced rock-basins, to extend 

 itself equally in every direction, might, it is true, be counteracted, in direc- 

 tions perpendicular to the sides of a cuboidal block of granite, upon which 

 basins were forming; by a varying resistance to disintegration, originating 

 in an internal concentric structure in the rock. But this counteraction 

 would merely have the effect of preventing the depth of the basins from 

 being equal to their diameters ; and would tend, rather, to increase, than 

 to diminish the regularity of their form. Further remarks on this subject 

 will be found in p. 336. 



How 



