§ 109. MARL. 205 



II. 



ROCKS AND THE PRODUCTS OF THEIR WEATHERING. 



109. The object of analyzing a rock, for agricultural 

 purposes, may be to estimate the total amount of its con- 

 stituents, or to determine its solubility and the readiness 

 with which it would be disintegrated and converted hito 

 soil on exposure to the air. The method of the analysis 

 would be much the same as that already described for the 

 analysis of soils. A qualitative examination should pre- 

 cede the quantitative one, in order to learn the best way 

 of bringing the mineral into solution, as well as what 

 substances arc to be separated and determined. 



The examination of the products of the weathering of 

 a rock should be conducted in the same manner as a regu- 

 lar soil analysis, by treating it with the same solvents in 

 the same succession. 



The three more important substances that come under 

 this head are marl, limestone, and clay, and some special 

 directions for the analysis of each may be useful. 



MARL. 



As this substance is used by the farmer in its natural 

 condition, it should be taken in a similar condition, for 

 analysis, viz., air-dried and unignited. If taken fresh 

 from the pit, it should be allowed to lie for a long time 

 exposed to the air on filter-ijaper, until thoroughly dry. 



The most useful of the determinations mentioned below 

 are those of phosphoric acid and the alkalies, and the me- 

 chanical analysis. 



The mechanical mialysls. For this, which is of much 

 importance, since the value of a marl usually depends 

 largely on the fineness of the division of its particles, 

 treat 30-50 grms., according to the amount of calcic car- 



