240 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jail. 18, 



chalk, in connexion with furrows, and their comparative rarity in 

 sand, gravel, and clay, in which furrows prevail, may be due to the 

 greater solidity of the chalk. It may also be partly due to its cal- 

 careous composition ; for, though I contend for the prevalence of 

 mechanical action in the formation of these cavities, I do not exclude, 

 that chemical action which water always exercises by means of the 

 carbonic acid which it absorbs. Lieut. Newbold * observed that, 

 while some of the rivers of India, in times of flood, excavated in their 

 rocky beds basins of the different shapes which we find in the chalk, 

 these cavities, when the river contracted its volume during the dry 

 season, were left filled with stagnant water, and this water, by its 

 chemical action, softened the gneiss in such a manner as greatly to 

 facilitate the boring process during the ensuing rainy season. Simi- 

 lar alternations between vorticose and stagnant water, the one acting 

 mechanically, the other chemically, would take place on the shore of 

 the sea ; and the greater liability of calcareous strata to be acted 

 upon by acidulated water, combined with the fact of the greater 

 prevalence of regular cavities in calcareous subsoils, warrants the 

 conclusion, that in their formation the solvent power of carbonic acid, 

 held in solution by stagnant water, has aided the operations of water 

 in violent motion. 



Neither would I wholly exclude subsidence of the matter ^\ith 

 which the cavities are filled, though I believe that the irregular 

 stratification over the larger cavities, which has been ascribed to this 

 cause, is generally an original condition of deposit upon an irregular 

 surface. The vertical striae on the sides of some pipes are proofs of 

 some amount of subsidence ; but this is not incompatible with the 

 formation of the pipes before the matter was deposited with which 

 they are filled. Subject, as they have been, to the pressure of many 

 hundred feet of strata, and many hundred fathoms of water, during 

 the submergence of the Crag and Erratic periods, it is not surprising 

 that there is evidence of subsidence to a certain extent, particularly 

 if a pipe, formed by the mechanical action of water, has been at all 

 deepened by the subsequent percolation of acidulated water. In a 

 former paper I have given a section of some faulted strata, over a 

 pipe, in which the greatest amount of displacement was 2 feetf . 



With regard to the pipes and furrows filled with warp-drift which 

 occur on the surface of various strata, I wish to offer no opinion 

 whether they were formed, as Sir Roderick Murchison has suggested, 

 by inundations analogous to those of earthquake-waves, or whether 

 they indicate some anomalous atmospheric action. In the latter 

 case, they may be compared to the excavations formed by torrential 

 rivers. All I contend for is, that they are due to some aqueous 

 operations, which were neither ordinary marine, nor ordinary atmo- 

 spheric action, and that these took place upon a terrestrial surface 

 which was inhabited by elephantine and other mammals now extinct, 

 after the desiccation of the bed of the Erratic or Glacial sea. 



* Proc. Geol. Soc. vol. iii. p. 704. 



t Quart. Journ. Geol. Soc. vol. i. p. 308, fig. 6. 



