Moreover, the nature and especially the adhesiveness of the clay substance as obtained by analysis may vary considerably in the presence of a very large amount of the finest grain-sizes; among which ferric hydrate or iron rust is especially apt to accumulate predominantly in the clay, considerably increasing its apparent weight and greatly diminishing its adhesiveness.[24] In strongly ferruginous soils, therefore, it becomes necessary to take into special consideration the amount of the ferric hydrate or rust which accumulates in the clay substance. The presence of large amounts of humus or vegetable mold also influences materially the adhesiveness and physical properties of the clay obtained by the method described, although most of it remains with the finer powdery sediments or grain-sizes. There are, besides, other colloidal or at least amorphous substances present in all soils, such as silicic, aluminic and zeolitic hydrates, which are all non-plastic, and yet sufficiently fine to form part of the “clay” obtained as above specified. [24] This fact emphasizes the impossibility of explaining the plasticity and adhesiveness of clay simply as a function of fineness of grain. Despite these imperfections, (which however can in a measure be taken into consideration in judging of a soil’s tilling qualities by its clay content), the figures given in the above table approximate much more nearly to a tangible basis for such estimate, than the utterly indefinite mixtures which under the older methods of analysis have been, and still are to some extent, used as a basis for soil classification by writers on agriculture. _Rock Powder; Sand, Silt and Dust._ The powdery (sandy and silty) constituents of soils usually constitute the greater part of their mass; and the proportions present of the several grades of fineness exert a most decisive influence upon their cultural qualities, and very commonly upon their agricultural value also. It is needless to add that the kind of mineral of which they consist or from which they were formed, is also of great importance in determining the quality of soils from the standpoint of the chemist, with respect to their content of mineral plant-food. WEATHERING IN HUMID AND ARID REGIONS. _Sands of the Humid Regions._—As has already been stated, “sand” is usually understood to be, in the main, quartz more or less finely pulverized, generally intermingled with a few grains of other minerals. With this understanding, since quartz is practically inert with respect to plant nutrition, it follows that soils consisting mainly of this substance contain but little plant-food; hence the common expression “poor, sandy land,” the outcome of the experience had in Europe and in the Eastern United States, and which until recently has been held to be of general application. The “sands of the desert” have, both in ordinary life and in poetry, always stood as the symbol of sterility.