WEATHERING OF ROCKS. 605 



northern side its influence being far greater, large masses of the limestone would 

 be dislodged and fall by mere gravitation into the gorge. The formation of these 

 Cliffs may have commenced before the Lovi'er Secondary strata were denuded in 

 the vale of Wrington, so that the stream which helped to shape them would have 

 run over an old plain of deposition or denudation, just as the Avon did when 

 it commenced to form the gorge at Clifton. 



The surface of the Carboniferous Limestone is frequently weathered into curious 

 hollows and fantastic shapes, used for Rustic work. These features are met with 

 in exposed situations, and are often found on the removal of the turf, near Cheddar 

 and other places in Somersetshire, near Denbigh, around Ingleborough, at Clints 

 Crags near Cockermouth, at Orton Scar near Appleby, etc. 



By far the larger number of holes and irregular cavities or honeycombed surfaces 

 in limestone are undoubtedly due to atmospheric (chemical and mechanical) wear 

 and tear, for in many cases fossils stand out in relief in the hollows ; but owing 

 to the presence of Helix tientora/is and other land-snails in them, some cavities 

 have been attributed to the action of Mollusca. Long ago Dr. Auckland, and 

 subsequently M. Bouchard-Chantereaux, advocated the boring powers of these 

 animals ; ' and Mr. John Rofe considers that the radula of snails, which is a 

 chitinous membrane bearing a long series of teeth, is capable of producing small 

 cavities in limestone rocks.- More evidence is perhaps needed on this point, 

 although, as snails may not always obtain from plants sufficient calcareous 

 matter for their shells, they may have some object in rasping away at limestone 

 rocks. Certainly they thrive best in districts where these rocks are developed. •* 



Curious irregular hollows are sometimes met with in the Great Oolite, and these 

 appear in many cases to be due to the weathering out of organic remains, such as 

 branching Corals or Sponges. (See remarks on Dagham stone,* p. 301.) Lichens 

 are also agents of disintegration, and sometimes originate small cavities in 

 stone. 



The occurrence of large masses of loose rock may be due to the jointage of beds 

 and their being weathered out in siui ; other rocks may jut out naturally and be 

 weathered into fantastic shapes, and not only may pluvial action influence them, 

 but in some cases wind carrying sand will exert great power in furrowing or in 

 polishing rocks. (See notes on Greywethers, p. 448.) Large blocks of stone may 

 also have been brought from a distance by a glacier or iceberg and so deposited. 

 (See p. 483.) On the other hand, boulders transported by Glacial agency and 

 deposited on the Carboniferous Limestone, are sometimes found on pedestals of 

 that rock, due to the protection locally afforded by the boulder to the subjacent 

 rock ; these are known as ' Pedestal Blocks.' ^ 



In areas formed of granite the hills are often surmounted by piles of stone like 

 boulders, for which they have been mistaken. The exterior of these stones, 

 originally quadrangular, acquires a rounded form by atmospheric action, for the 

 edges and angles of jointed-masses waste away more rapidly than the sides. Thus 

 spheroidal blocks are weathered out in situ, and often form Logans or Rocking 

 Stones.^ 



The Nutcracker, a Logan in Lustleigh Cleave, in Devonshire, is situated on the 

 side of the hill, and has rolled down from above. The best known Logan is that 

 situated in Cornwall, near Castle Treryn, St. Levan. It rocks only in one 

 direction, and a quantity of loose quartzose gravel may be found near the points 



^ Buckland, Proc. G. S. iii. 430 ; Bouchard-Chantereaux, Ann. des Sc. Nat. 

 (4), xvi. 197 ; Rev. John Hodgson, Hist, of Northumberland, Part 2, i. 193 ; 

 Sir W. C. Trevelyan, Edin. Phil Journ. xl. 396 ; Mackintosh, G. Mag. 1870, 

 p. 48 ; Bonney, Ibid. 1869, p. 483, 1870, 267, 1872, 315; Proc. Somerset Arch, 

 and Nat. Hist. Soc xix. 50 ; Hugh Miller, jun., Nat. Hist. Trans. Northumber- 

 land, vii. 18S0, p. 69 ; Miss E. Hodgson, G. Mag. 1867, p. 405 ; see also Ibid. 

 pp. 297, 404, and Hughes, Proc. Chester Soc. Nat. Sc, 1885, p. 19. 



2 G. Mag. 1870, p. 4. 



* See Meyer, quoted by Topley, Geol. Weald, p. 256. 



* S. P. Woodward, G. Mag. 1867, p. 405. 



* Hughes, Q. J. Ixii. 534. 



^ See T. R. Jones, Geologist, ii. 301 ; Sedgwick, Proc. G. S. ii. 184. 



