THE 



GEOLO&ICAL MAGAZINE. 



NEW SERIES. DECADE II. VOL. III. 



No. III.— MARCH, 1876. 



OI^IO-IZsTJ^Xi .a.i^tiox.:e]S. 



M 



I. — Sub AERIAL Denudation versus Glacial Erosion. 

 By W. GuNN, 

 Of H. M. Geological Survey of England and Wales. 



Y friend and colleague, Mr. Goodcliild, has contributed to the 

 Geol. Mag. for July, August and October last, articles on 

 Glacial Erosion in the Yorkshire Dale District, in order to evoke 

 criticism ; and as no one who knows the ground has come forward 

 to reply in detail to his arguments, I am induced to offer some notes 

 of my own on the subject.^ 



Much of both his articles is based on the supposed fact that 

 " under the influence of the weather, limestone, as is well known, 

 often disappears with great rapidity " (p. 326) ; and " disappears in 

 this way faster than any other rock" (pp. 327, 359, 362, 488, 491). 

 He says (p. 327) : " It will then readily be admitted by most geolo- 

 gists that under purely atmospheric conditions the rock that tends 

 to disappear the fastest is limestone ; next to this shale ; and the 

 slowest of all to weather away is sandstone." I, for one, cannot 

 admit this, nor do I think most geologists would admit it. Jukes 

 certainly thought the Carboniferous Limestone of Ireland wasted 

 faster in this Avay than the Old Eed Sandstone ; ^ but he says, " Any 

 bands of soft shales and clays would of course yield more rapidly 

 than the sandstones, or perhaps even than the limestones, and form 

 corresponding low lands and valleys."^ Dyell, writing of the 

 raised sea-beaches of the Gulf of St. Lawrence, 60 feet above present 

 highest tides, says : '' In the North American beaches above men- 

 tioned rounded fragments of limestone have been found perforated 

 by litliodomi ; and holes drilled by the same mollusks have been 

 detected in the columnar rocks or ' flower pots,' showing that there 

 has been no great amount of atmospheric decomposition on the sur- 

 face, or the cavities alluded to would have disappeared." * This 

 shows that limestone does not everywhere waste with great rapidity. 

 Further on the same author says, " Notwithstanding the enduring 

 nature of the marks left by littoral action on calcareous rocks, we 

 can by no means detect sea-beaches and inland cliffs everywhere, 

 even in Sicily and the Morea. On the contrary, they are, upon the 



^ The author wishes to state, that, when this article was written, the January 

 Numher had not appeared, and he was unaware of the fact that his colleague, Mr. 

 Hugh Miller, had written upon the subject (see Geol. Mag. January, 187t), p. 23). 



■^ Jukes and Geikie, Manual, third ed., p. 465. 



3 Op. cit. p. 456. * Elements, sixth ed., p. 78. 



DECADE II. — VOL. III. — NO. III. 7 



