I.] CARBONATE OF LIME 23 



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. 



Owing to its solubility, the weathering of limestone 

 takes the form of the removal of calcium carbonate 

 more or less completely, leaving a fine-grained residue 

 of the insoluble clay or sand. In the case of chalk 

 and of the purer limestones, the insoluble residue con- 

 sists mainly of a fine red or yellow clay; the chalk 

 downs, when not obscured by drift formations, are 

 covered with a sticky, reddish soil, only as a rule a 

 few inches in thickness, and though the actual chalk 

 is so close, in many cases this soil is almost deprived 

 of all its calcium carbonate. Almost exactly similar 

 material may be obtained in the laboratory by dis- 

 solving a few pounds of chalk or limestone in dilute 

 hydrochloric acid. Whenever a section is exposed in 

 chalk or limestone rocks, it will be noticed that the 

 dividing line between soil and rock is very irregular; 

 thin as the soil may be as a whole, in places it descends 

 into cavities and " pipes " in the rock, sometimes 20 or 

 30 feet deep. In these depressions the soil is the same 

 reddish clay as occurs on the surface, mixed with flints 

 in the case of the upper chalk ; they are essentially the 



