Action of the Atmosphere ^ipon newly -deepened Soil. 469 



Many sandstones are merely grains of quartz cemented together 

 by carbonate of lime. Many limestones again are but a mass of 

 organic remains, such as shells or corals. The mountain lime- 

 stone in England for hundreds of feet in depth often consists of 

 little else but remains of encrlnites, while the animal origin of 

 the white chalk seems equally obvious, the white calcareous 

 mud found around coral reefs, being when dried undistinguish- 

 able from common chalk. As many limestones are but remains 

 of corals the composition of these bodies becomes a matter of 

 interest. From many analyses of corals made at the Museum of 

 Practical Geology they were found to contain: — 



Carbonate of lime .. .. from 82" to 95'5 percent. 



Carbonate of magnesia .. ,, trace ,, 7*24 ,, 



Sulphate of lime .. .. ,, trace ,, 2'76 ,, 



Organic matter .. .. „ 3" ,, 8*27 „ 



Silica, alumina, iron, phosphates and fluorides were also obtained. 

 A little phosphoric acid has also been found in most limestones 

 that have been analysed with sufficient care. Messrs. Paine and 

 Way have also contributed a paper to the Journal in which they 

 point out the presence of considerable quantities of phosphate of 

 lime in the greensands above and below tlie gault. The green 

 grains of the sand are shown to contain sometimes 20 per cent. 

 of pliosphoric acid and a large proportion of lime, &c., and Prof. 

 Ilenslow stated in 1845 that " a stratum of greensand, although 

 never more than a foot thick, occurred near the surface over 

 many square miles in tlie vicinity of Cambridge, the pebbles in 

 it yielding 61 per cent, of earthy phosphates. The greensand is 

 a variable mixture of green grains with pure white particles of 

 quartz, the latter forming the largest proportion. From the 

 mean results of some careful analyses of oolitic limestones at the 

 Museum of Geology, they were found to contain — 



Water from 0-13 to I'^D i)er cent. 



Silica „ 0-22 „ 13-(J2 „ 



Protoxide of manganese .. ,, 0' ,, 1*08 ,, 



„ of iron .. .. „ 0* „ 1-13 „ 



Peroxide of iron .. •• ,, 0* ,, 0\S2 ,, 



Alumina „ 0" ,, 0-54 „ 



Lime „ 2^-30 „ o4-94 „ 



Magnesia „ 0-14 „ 21-42 „ 



Soda „ 0- „ 0-44 „ 



Carbonic acid „ 3()"08 ,, 4G*50 ,, 



Sulphuric acid ,, 0" ,, 0-12 ,, 



Phospiioric acid .. •• ,, 0' „ ^ trace ,, 



Chlorino ,, 0' ,, 0'03 ,, 



Those most liable to be afTc^cted by the atmosphere contained a 



