42 SOILS. 



of calcareous hills. When " hard " water, being usually such 

 as contains lime carbonate dissolved in carbonic acid, is boiled, 

 or long exposed to the air, carbonic gas escapes and the lime 

 salt is deposited partly on the walls of the kettle, partly form- 

 ing a pellicle on the surface of the water. 



Dolomite, or bitter spar, greatly resembles calcite in its as- 

 pect and properties, although containing nearly half its weight 

 (47.6%) of magnesic, together with calcic carbonate. It is, 

 however, nearly always whitish-opaque; its crystalline and 

 cleavage surfaces are usually somewhat curved ; and its effer- 

 vescence with acids is much less lively than in the case of 

 calcite. Like the latter it often forms pure granular rock de- 

 posits, frequently used instead of marble and limestone, and 

 under that designation. The dolomite rocks, however, are 

 much more subject to weathering than the non-magnesian lime- 

 stones, and it is a curious fact that in contradistinction to the 

 limestone regions proper, those having strongly magnesian 

 limestones or dolomites as their country rock are frequently 

 remarkably sterile. In some portions of Europe dolomite 

 areas are sandy deserts, whose sand consists of weathered do- 

 lomite, so pure as to offer no adequate supply of mineral food 

 to plants. In the United States, magnesian limestones under- 

 lie the "barrens" of several States and thus seem to justify 

 their European reputation of being poor soil-formers. The 

 exact cause of this difference is not fully understood, for at 

 first sight it is not clear why the presence of the magnesian 

 carbonate should interfere with the well-known beneficial ef- 

 fects of the lime compound. O. Loew and May l and others 

 have, however, shown that a certain excess of lime over mag- 

 nesia in the soil is necessary to prevent the injurious effects 

 exerted by magnesic compounds on plant nutrition, in the ab- 

 sence of an adequate supply of lime. This point will be dis- 

 cussed more in detail farther on. 



Sclcnite or Gypsum, sulfate of lime with about 14 per cent 

 of water, though not as abundant in nature as the carbonate or 

 limestone, is a very widely disseminated mineral and often 

 occurs in large masses over considerable areas. These are un- 

 doubtedly in most cases the result of evaporation of sea water 



1 Bull. No. I, U. S. Dept. Agr. Veg. Path, and Physiol. Investig. 



