340 Mr. Brayley, Jun., on the Connection of 



be expected to be found in the millstone-grit of this particular 

 spot. The immediate vicinity of trap, points out a cause for the 

 existence in it of such a structure ; reasoning on the analogy 

 of the case with those of the sandstones of Rum and Dun- 

 bar, as described by Dr. Macculloch*. It may also be ob- 

 served, in accordance with the phaenomena exhibited in those 

 localities, that the same denudation, which has exposed, at 

 Ashover, the millstone-grit and the shale, with the subjacent 

 limestone and toadstone, may have removed masses of the 

 rock last named, which may have been in contact with por- 

 tions of the millstone-grit, now also removed, and thus have 

 imparted, through their medium, a concretionary structure, to 

 the masses of grit which still remain ; and with which they 

 were once continuous. The writer, however, is unacquainted 

 with the particular features and history of this denudation; 

 nor are the authorities on the subject, at present, accessible 

 to him; he merely throws out these suggestions, therefore, 

 for the consideration of those geologists, who may have in- 

 vestigated the phaenomena presented by the valley of Ashover, 

 or who can refer to the authorities alluded to. 



Since the publication of the paper On the Granite Tors of 

 Cornwall, in which Dr. Macculloch first developed his views 

 respecting the interior spheroidal structure of granite, as in- 

 dicated by the manner of its decomposition, he has pursued 

 the subject much further, in the course of his admirable and 

 profound researches into the history of our unstratified and 

 crystalline rocks. He has ascertained some important limita- 

 tions to the validity of the inferences, which geologists had been 

 disposed to draw, from the desquamation, in concentric crusts, 

 of masses of granite and of trap. He has shown, that, — 

 while the mode of decomposition in question is in many cases 

 indubitably the result of an internal concretionary structure 

 in the decomposing rock, — in others it is utterly independent 

 on any structure; being produced, solely, by the action of 

 the atmosphere, by means, however, hitherto inexplicable ; 

 and that, in other instances, as in those, for example, from 

 which the spheroidal structure of granite had principally 

 been inferred, it is impossible to discover, from which of 

 these causes the process of desquamation in reality results f. 

 Since, however, in many cases, as just mentioned, the depend- 

 ence of the process on the interior structure in question, is 



* See his paper " On a Prismatic Structure in Sandstone induced by arti- 

 ficial Heat," &c., Quart. Journ. of Science, N.S. No. xii. pp. 263, 264. 



■f See Dr. Macculloch's paper " On the Desquamation of certain Rocks," 

 &c, Quart. Journ. of Science, vol xiii. p. 237. 



manifest, 



