38 Agricultural Chemistry. 



Apatite or crystalized phosphate of lime, is present in small 

 quantities in many of the older rocks, and is probably the original 

 source of the phosphoric acid of soils. Apatite also occurs mas- 

 sive in some of the older rock formations and is mined as a raw 

 material for the manufacture of phosphate manure in Norway, 

 Canada and particularly in some of our southern states, as Flor- 

 ida, the Carolinas, Georgia and Tennessee. 



A brief description of some of the more important rocks will 

 now be given. The igneous rocks are the oldest and it was from 

 the debris of igneous rocks that sand stones, shales and, indirect- 

 ly, limestones were formed. 



Sand stones and grits, consist of the larger fragments of the 

 waste resulting from the breaking up of igneous rocks, as for 

 example granite, which in consequence of their size and weight 

 have been deposited at or near the mouths of rivers. Their main 

 ingredient is silica, the grains of -sand consisting largely of quartz 

 crystals, but in many cases fragments of feldspars, mica and 

 other minerals are present. These grains are cemented together 

 either by calcium carbonate, as in calcareous sand-stones, by clay, 

 as in argillaceous sand-stones, by iron oxide, as in ferruginous 

 sand-stones, or by silica, as in siliceous sand-stones. Soils pro- 

 duced by the decay of sand-stones are light and friable and poor 

 in plant food unless there is present potassium-containing min- 

 erals as feldspar and mica. 



Shales consist principally of the plastic hydrated aluminum 

 silicate, kaolin, but may contain any other extremely finely di- 

 vided matter obtained by the erosion of the original rock. Par 

 tides of undecomposed or partially decomposed feldspars are 

 often present and these are of importance because of the potasli 

 they contain. Soils formed from shales are "heavy" and cl.-iycy. 

 generally sufficiently rich in potassium, but poor in phosphorus 

 and calcium carbonate (lime). 



Limestones, in which term chalk and magnesian limestone m;iy 

 also be included, have been formed largely by the abstraction 

 from water by living organisms, as coral polyps, shell fish, etc., 



