338 Mr. Brayley, Jun., mi the Connection of 



Another circumstance in the history of rock-basins appears 

 deserving of further inquiry. Although it is very true that 

 rock-basins do occur on the highly-inclined and even vertical 

 surfaces of granite, in some places, yet by far the greater num- 

 ber of them, in Cornwall and Devonshire at least, are found 

 either on horizontal or on gently-inclined surfaces, Has this 

 fact any connection, it may be asked, with the structure of the 

 rock? and does it indicate any determinate relation between 



rock ; we have no reason to think that its presence would materially in- 

 fluence the production of the concretionary spheroidal structure, by the 

 means just described ; since a very perfect concentric structure, radically 

 spheroidal, has been produced, in all probability by this species of action, 

 at Dunbar, in a rock quite devoid of felspar. 



It will be perceived that the foregoing qualification of the text has been 

 suggested, by a view of the origin of the structural phaenomena presented 

 by granite, somewhat different from that which is derivable from the in- 

 ferences of Dr. Macculloch, as given in his original paper on the subject. 

 The train of argument employed in that paper (" On the Granite Tors," &c.) 

 leads to the conclusion, that the interior spheroidal structure of granite 

 resulted from the process, of crystallization, by which the fluid, consisting of 

 the ultimate elements of that rock, in a state of igneous fusion, became 

 solid ; and in which, also, those elements were so associated together, as to 

 form the minerals of which granite is an aggregate. 



The facts and arguments, however, which Dr. Macculloch has subse- 

 quently brought forward, in his papers " On the Concretionary and Crystal- 

 line Structures of Rocks," and " On a Prismatic Structure in Sandstone in- 

 duced by artificial Heat," &c, some of which have already been mentioned 

 in this note, point to a modification of the conclusion just stated ; although 

 Dr. M. has refrained, in the latter paper at least, from extending his in- 

 ferences beyond the formation of prismatic trap, and has not connected the 

 subject, theoretically, with granite. 



From these facts and arguments, then, we should be disposed to infer, 

 that, while the crystalline-granular structure of granite— the crystallization 

 of its constituent minerals, their interference with each other's forms, and 

 the variety of relative proportions in which they are mingled, — has resulted 

 from the crystallization of the fluid while slowly cooling — the internal con- 

 centric-spheroidal structure, indifferently affecting all those minerals, is a 

 merely concretionary one resulting from the continued action of heat upon 

 the rock, subsequently to its consolidation. 



Jn concluding, for the present, this part of the subject, candour, how- 

 ever, requires a cautionary remark. Dr. Macculloch, in the two papers 

 last mentioned, has only once alluded to granite in connection with it, when 

 hinting, at the end of his account of the concretionary and crystalline 

 structures, that " the granitic laminae of the Alps.... have been produced 

 by a concretionary arrangement analogous [only] to crystallization." The 

 present writer, therefore, is alone accountable for the foregoing deduction 

 from the facts adduced by that gentleman; and if there be any error in 

 the extension of these views to the formation of granite, it is his only : 

 while if that deduction shall be found to throw any new light upon the, at 

 present, obscure history of that rock, the credit of it will be due, entirely, 

 to Dr. Macculloch, of whose arguments it is a mere application ; such as 

 the mind of every geological student, it is probable, would be disposed, 

 spontaneously, to make, after perusing the three memoirs which have been 

 quoted. 



the 



