L] CARBONATE OF LIME 23 



Olivine is essentially a silicate of magnesia and 

 protoxide of iron, not uncommon in some basalts, 

 which easily weathers and becomes a soft hydrated 

 silicate, called serpentine, to which talc is very similar 

 in composition. These magnesian silicates are not 

 of great importance in the British Islands; only in the 

 Lizard district of Cornwall are they extensively 

 developed and give rise to poor, barren soils. 



Calcium Carbonate, though present in many of the 

 older rocks in the crystalline form of Calcite or Iceland 

 Spar, is there to be regarded rather as a secondary product 

 brought by infiltering water than an original mineral. 

 It is soluble in water charged with carbonic acid ; 

 hence when the complex silicates containing lime 

 are weathered, the lime is removed in this form. The 

 calcium carbonate is redeposited when the water loses 

 the carbonic acid either by evaporation or by diffusion 

 on contact with air. In a massive form calcium carbon- 

 ate forms many of the sedimentary formations the 

 older ones hardened to limestones, and the more recent 

 ones soft like the chalk ; in these cases it has been secreted 

 from natural waters by living organisms, foraminifera, 

 corals, etc., and only gets a crystalline structure by 

 later change. Calcium carbonate from organic sources 

 is present to some extent in nearly all sedimentary 

 rocks ; the vast majority of the fossils there found are 

 constituted of calcite. 



In the limestone and chalk rocks the calcium car- 

 bonate is never quite pure ; in the white chalk, which 

 is the purest, the proportion of calcium carbonate, after 

 excluding the flints, is only about 98 per cent. ; in 

 others the proportion of clay and mud which were 

 simultaneously deposited gradually increases, so that 

 we can find rocks of every gradation between chalk and 

 clay or sandstone. 



