THE MAJOR SOIL-FORMING MINERALS. 43 



(see p. 26), more rarely of the transformation of limestone. 

 In mass it frequently resembles the latter, but is readily dis- 

 tinguished by its softness; it does not grit between the teeth, 

 is readily cut with a knife and does not effervesce with acids. 

 Very commonly it occurs in crystals, which are easily split into 

 thin plates. The crystals are very frequently found imbedded 

 in gray or bluish, tough clays, in rosettes, or flat sheets which 

 mostly show characteristic incurrent angles (caused by twin- 

 ning), and are hence known as " swallowtail " crystals. Such 

 sheets of selenite are popularly called " isinglass," which name 

 however is equally applied to the mineral mica (see p. 35). 



Gypsum is only exceptionally an abundant ingredient of 

 soils; yet such soils prevail quite extensively on the upper Rio 

 Grande, in New Mexico and adjacent portions of Chihuahua, 

 Coahuila, and on the Staked Plains of Texas. Here whole 

 ranges of hills are sometimes composed of gypseous sand, bear 

 a scanty, peculiar vegetation, and are ill adapted to agricultural 

 use. It may be said in general that few naturally gypseous 

 soils are very productive. This is largely because of the very 

 heavy clays which commonly accompany it, as the compound it- 

 self is not only not hostile to plant life but is in extended use as 

 a valuable fertilizer ("land plaster") for special purposes. 

 From causes not fully understood as yet, it particularly pro- 

 motes the growth of leguminous plants, notably the clovers; 

 and as stated in chapter 9, it also specially favors nitrification in 

 soils. In the arid region it renders important service in the 

 neutralization of " black alkali " or carbonate of soda in alkali 

 soils. Being soluble in 400 parts of water, it easily penetrates 

 downward in most soils, and in doing so effects changes in the 

 zeolitic portions, setting free potash from silicates and thus in- 

 directly supplying plants with this essential ingredient in a 

 soluble form. About 200 pounds per acre is an ordinary dose. 



For agricultural use the rock gypsum is ground in mills 

 so as to be easily distributed, and dissolved by the soil water. 

 Frequently, however, it occurs in the soft granular form 

 (gypseous marl) requiring only light crushing; thus in the 

 hills bordering the Great Valley of California, and in parts of 

 New Mexico and Texas. 



Iron Minerals. In connection with calcite and dolomite, the 



