240 GEOLOGY AND CHEMISTRY OF SOILS. 



The fertility of this soil depends upon the mixture of oth- 

 er earths, such as clay. The argillaceous slate soil is quite 

 productive, but generally this soil is more sandy and less fer- 

 tile than that from mica slate. 



5. Gneiss soil is very abundant in New England. It has a 

 pale yellow color, and is sandy and gravelly, indicating by 

 its appearance great sterility. This, however, is not always 

 the case ; the gneiss rocks contain large quantities of potash 

 in their feldspar, as well as argillaceous and siliceous sub- 

 stances. These minerals when reduced to the proper de- 

 gree of fineness make a very fertile soil. It is of two kinds, 

 the common and the ferruginous gneiss soil. The latter is of 

 a reddish color, in consequence of the peroxide of iron which 

 it contains. 



6. Granite soil does not differ essentially from gneiss. Both 

 are composed of quartz, feldspar and mica, and of course 

 yield all the mineral materials necessary to fertility. The 

 granite soil differs in its texture from coarse gravel to fine 

 sand. Dr. Dana regards all soils as composed essentially of 

 " granitic sand," that is, just such materials as granite and 

 gneiss rocks would produce by the ordinary process of disin- 

 tegration. These rocks yield all the earths necessary to the 

 highest degree of fertility. But their degree of fertility will 

 depend upon their texture and the sub-soil. When they are 

 underlaid with clay, or hard gravel, cemented together and 

 made water-tight, they may be made very fertile, because they 

 will then retain the soluble manures ; but if the substratum is 

 open gravel or sand, the soil itself gravelly or sandy, they are 

 too easily drained of moisture, and permit the soluble ma- 

 nures to infiltrate or leach throuo-h them. Gneiss and granitic 

 soils are better for Indian corn and grass than for the smaller 

 grains. 



7. Sienite soil differs from granite in containing horn- 

 blende instead of mica. Its structure is somewhat finer, and 



