[34] In driving a light carriage over the land represented by No. 643 above, after a light rain, the wheels gathered up so much soil within a hundred yards as to render it necessary to stop and chop it off the tires by means of a hatchet. This is a common experience in the black prairie lands of Texas. _Calcareous Clay Soils crumble on drying._—The heavy clay soils of some of the calcareous prairies of the Southwest, instead of contracting into a stony mass on drying, on the contrary resolve into a mass of crumbs, thus producing excellent tilth. This occurs even though the land may have been plowed when wet, and of course is a great advantage. The most striking exemplification of this peculiarity occurs in the heavy but profusely fertile “buckshot” clay lands of the Yazoo bottom, in Mississippi, where it is usual to plant corn and sweet potatoes in the semi-fluid mud left after an overflow, after turning a shallow furrow, then covering by turning another. To the onlooker it seems impossible that such plantings could be successful; but within a short time the muddy surface becomes a bed of crumbs (“buckshot”), forming a seed-bed not readily excelled by any made by artificial means. Hence, largely, the almost invariable success of crops in the Yazoo region. _Port Hudson Bluff._—The same clay produces a most unpleasant result at the foot of the Port Hudson bluff, where it crops out some feet above low water. When after a freshet the water level falls below this stratum, on drying the clay disintegrates into crumbs just as does the Yazoo buckshot soil; with the result that at the next rise, the loose mass subsides into the river as a flood of mud. Thus the foot of the bluff is being constantly undermined, and the falling of the bluff scarp has obliged the town above to recede many hundreds of feet from its original historic site. The exact proportions of lime carbonate necessary to produce this phenomenon, and its necessary relations to clay substance and other physical soil ingredients, yet remain to be investigated.[35] [35] Schübler (Grundsätzed. Agrikulturchemie, 1838) ascribes the crumbling of calcareous clay soils to the difference in the contraction of calcareous sand and the clay substance. But it is doubtless more directly connected with the flocculation of the latter by lime. _Loamy and Sandy Soils._—It is largely the _absence_ of these extreme changes of volume that renders the cultivation of loamy or even sandy lands so much more easy, and the success of crops so much more safe, than is the case in clay soils. Whenever the content of colloidal clay diminishes below 15%, the shrinkage in drying from the wet condition becomes so slight as to cause no inconvenience; while in sandy soils properly speaking, no perceptible change in volume occurs.