558 APPENDIX B. 



able by its glassy luster and the irregular fracture absence of definite 

 planes or facets of cleavage, causing the grains to be abraded or rounded 

 nearly alike in all directions. The feldspars : , on the contrary, always show 

 a tendency to cleave into fragments having definite, obviously oblique 

 angles, which are perceptible even when the grains are somewhat worn ; 

 because of the difference in the ease with which wear takes place in the 

 several directions. Potash feldspar, moreover, which is the most im- 

 portant to be recognized because it indicates a relatively large supply 

 of potash in the natural soil, is but rarely glassy in luster, but mostly 

 dull white, or reddish-white. The lime and lime-soda feldspars rarely 

 show as definite forms, because of their tendency to form complex 

 crystalline aggregates (twins) : and their definite recognition requires 

 somewhat complex (polarizing) appliances in connection with the mi- 

 croscope. In such cases, however, the accompanying minerals (horn- 

 blende, pyroxene, mica and others) often afford valuable indications, 

 because of their known association with soda-lime feldspars in certain 

 rocks. 



An abundant occurrence of hornblende fragments, characterized by 

 their flat, tabular form, and bottle-green or black tint, indicate, almost 

 always a fairly good supply of lime in the soil, but leaves the potash- 

 supply in doubt. Pyroxene (distinguished by its smooth, polished sur- 

 face from the angularly-weathering, usually rusty hornblende fragments) 

 rarely occurs with potash feldspar; and soils strongly charged with it 

 are mostly poor in potash. 



Mica occurs in so many rocks and is of so little consequence as a 

 soil ingredient from the chemical point of view, because of its difficult 

 decomposition, that its presence can mostly only serve to corroborate 

 or contradict conclusions as to the derivation of a soil from some par- 

 ticular rock or region. But mica serves a good purpose in improving 

 the tilling qualities of soils. Its thin scales must not be mistaken for 

 the tabular fragments of hornblende. 



Calcite in its several forms is mostly easily recognized both by its 

 form under the microscope, and by the effervescence its granules show 

 when touched with an acid. This effervescence can generally be ob- 

 served on touching the wetted soil with chlorhydric acid, so soon as the 

 content exceeds two per cent ; but something depends upon the size of 

 the grains, as when these are very small, the giving-off of gas is less readily 

 noted. To facilitate it, the wetted soil may be warmed before touch- 

 ing it with the acid. The recognition of the presence of lime carbon- 

 ate in soils is so important as to justify considerable trouble in render- 

 ing it definite. When the aid of a chemist cannot be commanded, 



