564 Sharp — On a Singular Incrustation, 



A similar layer of calcareous paste, one foot in thickness, overlies 

 the mass, and has partially filled the interstices of the incrusted 

 plant. The whole is covered in with surface soil of the thickness 

 of one foot nine inches, the depth of the general surface soil over- 

 lying the gravel being only nine inches, but the surface level being 

 the same. 



The gravel is stratified, showing that it has remained undisturbed 

 since it was originally deposited, and the strata run up to the mass 

 of incrustation on either side, abutting sharply upon it, and were 

 evidently once continuous. 



These are the simple facts which I ascertained upon examination 

 of the mass of incrustation in situ, and upon observation of the sur- 

 rounding circumstances ; and I may, perhaps, be permitted to suggest 

 briefly what may have been the history of its formation. 



Ages before the ground was first broken to make the present 

 gravel pit — at a period perhaps as remote as that when savage man 

 roamed over the country, an excavation was made at this spot (pro- 

 bably to obtain water) and a pool was formed, about ten feet across, 

 and about five feet deep, in which grew luxuriantlj^ a water plant, 

 which Mr. Carruthers, F.L.S., F.G.S., of the British Museum, has 

 kindly informed me was the Cliara vulgaris of Linnseus. 



Eain, upon falling on the surface, soaks through the soil, and per- 

 colating the porous rock beneath, descends by its own gravity until 

 stopped by the occurrence of a clay or other impervious bed : it then 

 accumulates and travels until it finds vent at favourable points of the 

 surface, and issues thence in the form of springs. 



It is not an uncommon thing for water, charged with carbonic 

 acid, to dissolve out and hold in solution a certain amount of lime 

 from any limestone which it may encounter in its underground 

 flow ; but, upon becoming exposed to the atmosphere, the carbonic 

 acid would fly off in the form of gas, and the power of the water to 

 hold the lime in solution being thus impaired, the surplus carbonate 

 of lime would be precipitated, and collect round and solidly incrust 

 any nucleus, even a growing plant, which might happen to be in the 

 water. 



Withering describes two species of CJiara (C. Mspida and C. vulgaris) 

 — which become covered with a stony crust, because, as he says, 

 " they possess the property of absorbing carbonic acid gas, by which 

 lime has been held in solution, in a greater degree than any other 

 water-plant ; " and this would enhance the tendency in the water in 

 which such plants might grow to deposit lime, so far as regards the 

 incrustation of those plants. 



The gravel contains many fragments of the limestone of the district ; 

 and I found in it pieces which apparently had been exposed to the 

 dissolving agency described. 



The Chara, then in its rich and matted growth, in the pool of water 

 thus impregnated with lime, became incrusted — first at the bottom, 

 and gradually upwards — as the plant, still accumulating in its growth, 

 approached nearer and nearer to the surface. 



By this slow process of growth and incrustation, the pool, in the 



