Book III. THEORY OF DRAINING. 691 



of agriculture ; but where they are of a more tenacious and impenetrable quality, they 

 only become gradually softened by the stagnant water below them ; by which the surface 

 of the ground is, however, rendered equally moist and swampy, though somewhat more 

 slowly than in the former case. It may also be observed, that some of the strata which 

 constitute such hilly or mountainous tracts are found to be continued with much greater 

 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.) 



4215. These elevated strata frequently ]>rove the means of rendering the grounds below 

 wet and swampy ; for 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 materials, readily insinuates 

 itself, and thus passes along between the first and second or still more inferior strata 

 which compose the sides of such elevations, until its 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 appear- 

 ances 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 in 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 discharge, 

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

 (Johnston s Account of Ellingtons Mode of Draining Land, p. 15.) 



4216. 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 they are more frequently found to proceed from the contiguous 

 elevations into the low grounds that immediately surround them. 



4217. The naiure of the stratum of materials on which the ivater descending from liills has 

 to proceed must considerably influence its course, as well as the effects which it may 

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

 of the clayey, stiff marly, or impervious rocky kind, and not interrupted or broken by 

 any other materials of a more porous quality, the water 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. 



4218. These sorts of strata extend to very different depths in different situations and 

 districts, as it 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 kind, 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. 



4219. 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 considerably more than a hundred. 

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

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



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