1 84 SOIL - 



formed a subsoil-mass more or less firmly cemented by the dis- 

 solved matters, sometimes to the extent of stony hardness 

 (lime carbonate in the arid regions, kankar of India), more 

 usually soft enough to be penetrated by the pick or grubbing 

 hoe, and sometimes by the stronger roots of certain plants; 

 but resisting both the penetration and the assimilation of plant 

 food by the more delicate feeding roots. 



Nature of tJic Cements. The nature of the cements that 

 serve to consolidate the hardpan mass is substantially the same 

 as those already mentioned in the discussion of sandstones 

 (chapt. 4, p. 55) ; with the addition of those formed, usually 

 in connection with siliceous solutions, by the acids of the humus 

 group. The latter class of hardpans is especially conspicuous 

 in the case of swampy ground and clamp forests, where " moor- 

 bedpan " and reddish " ortstein " (the latter particularly devel- 

 oped in the forests of northern Kurope, where it has been 

 studied in detail by Miiller and Tuxen 1 . are characteristic. 

 The latter gives for a characteristic sample of the reddish hard- 

 pan underlying a beech forest in Denmark a content of from 

 2. 2O to 4.40% of ulmic compounds, and shows that the color 

 is due to these and not, as had been supposed, to ferric 

 oxid, which is present only in minute quantities 



Bog ore, Mi>orl>e<if>an. Ortstein. It is otherwise with moor- 

 bedp'in. which often consists of a mass of bog iron ore per- 

 meated or less with humous substances, which impart to it the 

 dark brown tint so often seen also in the " Mack gravel " spots 

 of badly-drained land. On the whole, however, ferric cements 

 are much less frequently found in hardpans than in sandstones 

 formed above ground. 



Clu\ substance washed from the surface into the subsoil by 

 rains (chapter 10, p. ini ) always helps materially to render 

 the hardpan impervious when afterwards cemented, a much 

 smaller proportion of the cementing material sufficing in that 

 case to form a solid laver. In such cases however the cement 



m> 



is rarely of a calcareous nature, --mce lime prevents the diffu- 

 sion and washing-down of the clay. It is mostly siliceous or 

 zeolitic; if the former, acid will have little or no effect upon 

 the solidity of the hardpan; while if zeolitic. acid will pretty 



1 See ' Studien iiber die natiirlichen Humusformen," by Dr. P. E. Miiller. 



