50 SCIENCE AND SOIL 



Gneiss may contain constituents not always present in granite, 

 because of admixture of other materials in transportation; also 

 certain constituents are likely to be lost to some extent in the origi- 

 nal disintegration and transportation, and to a great extent in the 

 subsequent more complete decomposition, so that often certain 

 constituents may show higher percentages in the final residue. 



Zeolites. Zeolites are formed from partially decomposed min- 

 erals, like granite and gneiss. They are hydrated double silicates 

 of aluminum with calcium or sodium, and may contain other bases, 

 especially potassium. They are credited with important functions 

 in soils to which further reference will be made. 



Shale, kaolin, and clay. These materials consist chiefly of hydrated 

 aluminum silicate related to the mineral kaolinite, Al 2 Si 2 O 5 (OH) 4 , 

 and representing in part the final residue from the decom- 

 position of felspar, hornblendes, micas, etc., from granite, gneiss, 

 and other silicate rocks. They may be grouped under the general 

 term argillites (from argil, meaning potter's clay) . Slate is the well- 

 known roofing material. Shale is the term applied to the more 

 thinly stratified formations which disintegrate more or less 

 readily when exposed to the weather. Kaolin is common fire clay. 

 Ordinary brick clay belongs in the same group, and, in fact, shale 

 itself is often ground and used for making brick or tile. 



Aluminum silicate is the final residue from the disintegration of 

 many different rocks, and consequently is itself one of the most 

 permanent substances. The oldest records of man have been pre- 

 served in burnt clay, both in tablets and in pottery. 



Carbonates. The carbonates include a very important group of 

 rocks, although they constitute a small portion of the earth's 

 crust when compared with the silicates. Of the carbonates, the 

 common limestone, calcium carbonate, CaCO 3 , is by far the most 

 abundant. It is frequently quite impure. Marble is calcium car- 

 bonate, mottled or colored with impurities and of sufficiently close 

 texture to admit of polishing. The mineral calcite is very pure 

 crystallized calcium carbonate, CaCO 3 . Magnesian limestone (dolo- 

 mite) is a double carbonate of calcium and magnesium, CaMg(CO 3 ) 2 , 

 but this compound is frequently mixed with calcium carbonate, 

 CaCO 3 , so that varying percentages of calcium and magnesium 

 are found in dolomitic limestone. 



