﻿340 THE EEY. E. C. SPICEE OX [^^g- IQoS, 



A stalactite was recently remoYed, from the roof of the Grand- 

 Junction Canal tunnel passing through the limestone near Ellsworth, 

 which was 10 feet long and more than a foot in diameter. This 

 had formed at one point in 100 years. 



The mean amount of dissolved material carried down hy the 

 Thames is 1502 tons daily, or 548,230 tons annually.^ If only 

 800 tons of this whole amount removed be taken as consisting 

 of carbonate of lime, it is estimated that 140 tons of this material 

 alone are removed yearly from each square mile ; while, if the 

 whole amount of 1500 tons daily be reckoned, the amount removed 

 is over 260 tons annually per square mile, which represents 3900 

 cubic feet annually, or 390,000 cubic feet per century from every 

 square mile.^ This material is not removed broadly and equally 

 over the area, but by streams issuing at definite spots in connexion 

 ■with, valleys in every case, where accordingly the resultant surface- 

 modification is localized. 



There are roughly 450 feet of permeable strata in the Great and 

 Inferior Oolite, above 540 feet of impervious Fuller's Earth and 

 Lias. An investigation of the Cotteswold rocks recorded by 

 Mr. Wethered in Quart. Journ. Geol. Soc. vol. xlvii (1891) p. 559 

 et seqq., shows that there is less than 1 per cent, of insoluble 

 matter in the purest beds, and over 67 per cent, in the argillaceous 

 beds. This insoluble residue is chiefly quartz ; then come felspars, 

 zircon, rutile, and tourmaline. At Leckhampton, for example, 

 there is 94 per cent, of carbonate of lime alone in the Great Oolite 

 rock. It seems a most remarkable circumstance that so many 

 people have performed these striking laboratory -experiments with 

 their consequent calculations ; and yet when they have seen the 

 deeply-indented valleys peculiar to the limestone and Chalk-regions 

 have always, inevitably and universally, with, so far as I can 

 discover, no single exception, considered them as the results of 

 surface -erosion, or else have confined themselves to digging grate- 

 fully fossils out of their sides. An early writer ^ invokes mechanical 

 action, in various forms, to account for Chalk- valleys ending in pot- 

 holes : breakers on the shore, torrential rivers, the inundation of 

 earthquake-waves, swirls of torrent-water, and so forth ; and, 

 although Prestwich immediately corrects him by referring them to 

 solution, and on p. 223 of the same volume of the Quarterly Journal 

 gives some remarkable solution-results, he, in a most extraordinary 

 way, just misses their application to valley-formation. Thence it 

 is a far cry to the latest memoir of the Geological Survey, in which 

 the Evenlode Valley is still regarded as the result of a denudation- 

 scour. 



The base-level of erosion in impervious rock is reached when a 

 river is graded, but the slowest and clearest meandering stream fed 

 from pervious soluble rocks is still a degrading agent of extra- 



^ J. Prestwich, Quart. Journ. Geol. Soc. vol. xxviii (1872) p. Ixrii, &c. 

 2 See Sir Archibald Geikie's ' Text-Book of Geology ' 3rd ed. (1893) pp. 378- 

 79. 



^ J. Trimmer, Quart. Journ. Geol. Soc. vol. x (1854) p. 238. 



