Correspondence. 333 



No one would uphold such, an idea. 



It follows then that while the beds were being tilted up, or since 

 they have been tilted, a sufficient mass of rock has been removed to 

 allow of the edges of some beds appearing at the surface that were 

 once buried many thousand feet below it. 



In other words, the present surface of the ground has been 

 gradually arrived at by the external removal of vast masses of rock 

 that previously covered it. 



It makes no difference whether the surface be a perfect plane or a 

 corrugated mountain chain, wherever beds crop to the surface, it can 

 only be the effect of denudation. 



A plain formed across the edges of a great series of beds, shows 

 that either from length of time or other circumstances the external 

 denuding force has completely obliterated the effect of the action of 

 elevation. 



A mountain chain shows that along a certain band of country the 

 action of elevation has been so great, or so long continued, that the 

 forces of denudation have not been able to overcome it. 



The mountains still stand in spite of the denudation, although 

 probably it has been much greater there than in the surrounding 

 regions, and therefore the most deeply-seated rocks have succeeded 

 in reaching the surface there. 



In every case the " form of the ground, " whether in mountains, 

 hills, valleys, or plains (except those so recently made that there has 

 been no time to modify it), is the result of external action upon 

 materials variously prepared and placed for it by internal force. 



I should suppose that the Alps must have concealed in them one 

 or two or more old surfaces, on which the superior formations repose 

 tmcomformably. Whether it will ever be possible to distinguish 

 these I do not know, but it must be done before the structure of the 

 mountains can be understood, or their history be unravelled. Careful 

 sections, on a true scale, with nothing inserted that cannot be 

 actually seen, must also be constructed before we can be said to be 

 in possession of even sufficient data to state the problem of Alpine 

 geological history. 



Lastly, allow me to say that I feel sure there is no real difference 

 between Mr. Poulett Scrope and myself in our opinions on this 

 subject, and that any apparent difference arises from the want of a 

 precise settlement of the meaning of terms. 



J. Beete Jukes. 



P.S. There is one passage in Mr. Mackintosh's letter on which I 

 may usefully make a remark, and that is at page 282, where he speaks 

 of the Old Red Sandstone of Herefordshire as more easily eroded than 

 the Carboniferous Limestone. The Old Eed Sandstone of Cork and 

 KeiTy , however, is mu.ch harder than that of Siluria, the shales being 

 all converted into hard clay-slates by true slaty cleavage. Many 

 parts of the Old Eed of the south-west of Ireland indeed are litho- 

 logically very like parts of the Cambrian rocks of North Wales and 

 Wicklow. 



KiLRONAN, Abban Island, Galway Bay, June Wth, 1866, 



