24 THE ORIGIN OF SOILS [chap. 



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 

 results of solution, and represent the lines along which 

 the drainage of the rain water has been more active, 

 owing to a joint or fissure in the rock below. 



Other minerals which do not constitute any large 

 proportion of the earth's crust, but still play some 

 part in the soil, are apatite, glauconite, selenite, limon- 

 ite, and iron pyrites. 



Apatite, or crystallised phosphate of lime, 

 Ca 5 (P0 4 ) 3 F, is present in small quantities in many 

 of the fundamental rocks, and is probably the ultimate 

 source of the phosphoric acid of soils. Apatite also 

 occurs massive in some of the older strata, and has been 

 worked as a raw material, for the manufacture of phos- 

 phatic manures, in Norway and Canada. 



