26 



SOILS: PROPERTIES AND MANAGEMENT 



This acidity and basicity is determined by the presence 

 of silica and the alkalies, respectively, as carried by 

 certain essential minerals. Suppose we name some 

 representative igneous rocks in the order of their acidity, 

 and list some of the minerals carried by them : 



1. Granite . . Quartz, orthoclase, and mica 



2. Diabase . . Plagioclase, mica, hornblende, or augite 



3. Peridotite . Principally olivine 



It is to be seen that the minerals contained by granite 

 are more resistant than those carried by either the diabase 

 or peridotite, while the olivine of the last group is near 

 the foot of the list when the minerals are arranged in the 

 order of their resistance. 



The following data 1 bear out the argument presented 

 above as to the relative resistance of rocks : 



PROPORTIONAL AMOUNTS OF FRESH ROCKS SOLUBLE IN BOIL- 

 ING HYDROCHLORIC ACID AND SODIUM CARBONATE SOLUTIONS 



It is evident, then, that the law of mineral resistance 

 applies to rocks as well as to the separate minerals, al- 

 though its application thereto is much more complex 

 and difficult to interpret. 



1 Merrill, G. P. Rocks, Rock Weathering, and Soils, p. 366, 

 New York. 1906. 



