82 



THE WONDERS OF GEOLOGY. Lect. I. 



possible to fire a pistol at the bottom of the cavern ; for, 

 though gunpowder may be exploded even in carbonic acid 

 by the application of a heat sufficient to decompose the 

 nitre, and consequently to envelope the mass in an atmo- 

 sphere of oxygen gas, yet the influence of a mere spark 

 from steel produces too slight an augmentation of tempera- 

 ture for this purpose.* • 



41. Consolidation by iron. — Water charged with a 

 large proportion of iron, acts an important part in the con- 

 solidation of loose materials, converting sand into iron- 

 stone, and beach or shingle into a ferruginous conglomerate. 

 On Clapham Common, and in other places in the vicinity 

 of London, large blocks of a very compact breccia occur, 

 being masses of chalk-flints more or less broken and 

 rolled, cemented together by an infiltration of iron. In 

 this example of a horse-shoe firmly impacted in a mass of 

 pebbles and sand, from the sea-beach at Eastbourn, the 

 cement which binds the mass is derived from the iron. 

 Nails are frequently found in the centre of a nodule of hard 

 sandstone formed by this process ; the nail having supplied 

 the water with the material by which the surrounding sand 

 has become concreted into stone. I have here a cannon ball 

 imbedded in the centre of a nodule of iron-stone, in which 

 are several oyster shells : this specimen was dredged up off 

 the Sussex coast, and has evidently been consolidated by 

 the partial oxidation of the iron. 



In this mass of breccia, which has been produced by 

 a like process, are two silver pennies of Edward I ; 

 it was found in 1832, at a depth of ten feet in the bed 

 of the river Dove, in Derbyshire. The coins are presumed 

 to be part of the treasures contained in the military 

 chest of the Earl of Lancaster, which was lost in this 

 stream in 1322 ; the soldiers being alarmed by a sudden 



* Daubeny on Volcanoes. 



