626 PRACTICE OF AGRICULTURE. Part III. 



regularity than others ; those which are placed nearest to the surface at the inferior parts 

 of such hills or elevations, being mostly broken or interrupted before they reach the tops 

 or higher parts of them ; while those which lie deeper, or below them at the bottom, show 

 themselves in these elevated situations. Thus, that stratum which may lie the third or 

 fourth, or still deeper, at the commencement of the valley may form the uppermost layer 

 on the summits of hills or mountainous elevations. This arrangement or distribution of 

 the different strata may have been produced partly by the circumstances attending the 

 original elevation of such mountainous regions, and partly from the materials of the 

 original exterior strata being dissolved and carried down into the valleys by successive 

 rains and other causes, and thus leaving such as were immediately below them in an ex- 

 posed and superficial state in these elevated situations. {Darwin s Phytologia, p. 258.) 



8911. These elevated strata frequently prove the means of rendering the grotmds below 

 wet and swampy ; for, from the night dews, and the general moisture of the atmosphere, 

 being condensed in much greater quantities in such elevated situations, the water thus 

 formed, as well as that which falls in rain and sinks through the superficial porous ma- 

 terials, readily insinuate themselves, and thus pass along between the first and second, or 

 still more inferior strata which compose the sides of such elevations ; until their descent is 

 retarded or totally obstructed by some impenetrable substance, such as clay; it there 

 becomes dammed up, and ultimately forced to filtrate slowly over it, or to rise to some 

 part of the surface, and constitute, according to the particular circumstances of the case, 

 different watery appearances in the grounds below These appearances are, oozing 

 springs, bogs, swamps, or morasses, weeping rocks from the water slowly issuing in 

 various places, or a large spring or rivulet from the union of small currents beneath the 

 ground. This is obvious from the sudden disappearance of moisture on some parts of 

 lands, while it stagnates, or remains till removed by the effects of evaporation on others ; 

 as well as from the force of springs being stronger in wet than dry weather, breaking out 

 frequently after the land has been impregnated with much moisture in higher situations, 

 and as the season becomes drier ceasing to flow, except at the lowest outlets. The force 

 of springs, or proportion of water which they send forth, depends likewise, in a great 

 measure, on the extent of the high ground on which the moisture is received and detained, 

 furnishing extensive reservoirs or collections of water, by which they become more amply 

 and regularly supplied. On this account, what are termed bog-springs, or such as 

 rise in valleys and low grounds, are considerably stronger and more regular in their dis- 

 charge, than such as burst forth on the more elevated situations or the sides of eminences. 

 (Johnston's Account of Elkington" s Mode of Draining Land, p. 15.) 



3912. The waters condensed on elevated regions are sometimes found to descend, for a 

 very considerable distance, among the porous substances between the different conducting 

 layers of clayey or other materials, before they break out or show themselves in the 

 grounds below ; but it is more frequently the case to find them proceeding from the con- 

 tiguous elevations into the low grounds that immediately surround them. 



391 3. The nature of the slratuin of materials on which the water proceeding from hills has 

 to penetrate, must considerably infiuence its course, as well as the effects which it may 

 produce on such lands as lie below, and into which it must pass. Where it is of the 

 clayey, stiff marly, or impervious rocky kinds, and not interrupted or broken by any other 

 kind of materials of a more porous quality, it may pass on to a much greater distance, 

 than where the stratum has been frequently broken and filled up with loose porous 

 materials, in which it will be detained, and of course rise up to the surface. 



3914. These sorts of strata extend to very different depths in different situations and dis- 

 tricts, as has been frequently noticed in the digging of pits, and the sinking of deep wells, 

 and other subterraneous cavities. The clayey strata are, however, in general found to 

 be more superficial than those of the compact, tenacious, marly kinds, or even those of a 

 firm, uninterrupted, rocky nature, and seldom of such a great thickness; they have, 

 nevertheless, been observed to vary greatly in this respect, being met with in some places 

 of a considerable thickness, while in others they scarcely exceed a few inches. 



3915. The intervening porous substances, or strata where clay prevails, are found, for the 

 most part, to be of either a gravelly or loose rocky nature. Stiff marly strata, which 

 approach much to the quality of clay, though in some instances they may present them- 

 selves near the surface, in general lie concealed at considerable depths under the true 

 clayey strata, and other layers of earthy or other materials ; they have been discovered 

 of various thicknesses, from eight or ten feet to coni?iderably more than a hundred. 

 {Darwin's Phytologia, p. 259.) The intervening materials, where strata of this nature 

 are predominant, are most commonly of the more sandy kinds ; possessing various degrees 

 of induration, so as in some cases to become perfectly hard and rocky, but with frequent 

 breaks or fissures passing through them. The loose, friable, marly strata are capable of 

 absorbing water, and of admitting it to filtrate and pass through them. 



3916. Thus the valleys and more level grounds must cottstantly be liable to be overcharged 

 with moisturef and to become, in consequence, spouty, boggy, or of the nature of a morass. 



