6^ Mackintosh— The Sea v. Bain and Frost. 



rows or ridges, whicli can only he explained by reference to similar 

 accumulations now thrown up by the sea on the coasts of Caithness, 

 or among the Shetland Islands. 



Bain incapable of abrading hard Bochs. — Mere rain, before it has 

 assumed the shape of running water over incoherent materials so as 

 to furnish it with solid artillery, is possessed of very little abrasive 

 power on hard rocks. Eain comes from the atmosphere quite un- 

 provided with the means of abrasion. Mr. Geikie,' admits that 

 uncharged water, even when it dashes with the force of the waves 

 and currents of the sea, is unable to abrade rocks. This accounts 

 for many inland rocky projections and pillars retaining the forms 

 with which they rose above the sea. In an article on the Brimham 

 Eocks,''* and in a paper read before the British Association at Bir- 

 mingham last year, I showed that the Millstone-grit rocks of York- 

 shire have in many instances not been abraded by rain or weathered 

 by any kind of subaerial agency since the last emergence of the 

 land. It is true that the windward side of certain kinds of sandstone 

 and other rocks frequently present the appearance of recent atmo- 

 spheric decay. But a little experience will enable one to distin- 

 guish between a chemically -corroded, frost-eaten, or rain-worn surface 

 of rock, and a rock abraded by the sea. The former is rough, 

 pitted, exfoliated, or fretted (as may be seen in the walls of our 

 New Red Sandstone churches), without any determinate shape being 

 produced ; the latter (with the exception of limestone rocks chemi- 

 cally acted on by the sea) comparatively smooth with more or less 

 of a regular form or continuous outline. In most places the Mill- 

 stone-grit of Yorkshire and Derbyshire presents few or no signs of 

 pluvial abrasion. On the contrary, the joints and crevices are often 

 filled up (as at the Black or Stonnis rocks, near Matlock) with a 

 deposit distinct from powdered gi'it, and resembling the drift of the 

 neighbouring valley. Mr. Hull has found that the Millstone-grit 

 projections of the Peak District retain their wave-worn forms ; and 

 if, at an elevation of about 2.000 feet above the sea, rain and frost 

 have not been able to lower the rocky surface many inches since the 

 close of the last submergence, would it not be rash to assign to them 

 a power equivalent to the denudation of the surrounding area ? 



But rain has not only been powerless among the rocks of the 

 north of England. Among the granitic tors of Dartmoor, what is 

 commonly called tveathering would appear to be very limited in its 

 effects. In many instances one side of a tor would appear to have 

 "been stripped bare by a very powerful laterally-operating cause, 

 whilst the blocks and fragments were scattered on the other side. 



* Scenery of Scotland in connection with its Physical Geology. The descriptive 

 powers of this accomplished geologist render him as worthy to be an illustrator of 

 Playftiir, as Playfair was of Huttou. 



2 Geol. Mag., vol. ii., No. 4. After the reading of the paper at Birmingham, 

 the President of the Section, Sir 11. I. Murohison, expressed his entire, concurrence 

 with the author's views in reference to the sea-shore origin of the Brimhara Rocks, 

 and the resistance tliey have o ered to the atmosphere. He likewise referred to the 

 preservation of glacial-markings on rocks as a proof of limited atmospheric denuda- 

 tion. 



