362 APPENDIX 



In order to understand precisely the nature of springs, it is necessary to distinguish 

 three different types: those in permeable soils, which are called "thalwegs"; those in 

 stratified soils; and those in soils where fissures abound. 



Let us take the case of a surface composed of permeable ground which is perfectly 

 horizontal; under this the water which filters through wiU form a sheet of which the 

 surface will be level, and will approach more nearly to the surface of the soil, according 

 to whether the precipitations have been more abundant or more recent. 



This point established, then if the plain happens to be approached by a valley the 

 latter will produce upon it an effect of drainage so that its level will become lower the 

 nearer it approaches the valley. If, at the base of this valley there is a stream of water 

 running, this stream will be fed by the sheet; thus it is that in all valleys composed of 

 permeable soils, to the right and left of the river bed, similar sheets of water are to be 

 found, and these will feed it, and do not always originate from the infiltration of its 

 waters, as has been said.^ 



If the surface of the soil is uneven the subterranean sheet will have an undulated 

 surface, reproducing in an attenuated form the unevenness of the ground. One sees 

 that in such soil the depth of wells will attain its maximum in the ridges, its minimum 

 in the thalwegs. 



It is this fact that has been perfectly verified in the sands of the Sahara, for instance, 

 and in the plains of permeable soil in the basin of the Seine. 



So long as the depressions in the soil do not reach as far down as the sheet of water 

 formed by infiltrations the latter has no possible outlet. But as soon as it comes in 

 contact with the bottom of a "thabveg" it overflows in springs of a kind which are 

 only found in "thalwegs." These springs, which are called "sommes" in Champagne, 

 are rarely perennial; they go up and down the valleys to the length of many kilometers 

 according to the oscillations of the subterranean sheet of water. In Picardy, a province 

 of notably permeable soil, the springs formerly came to light at a very considerable 

 distance up the river from their present point of emergence, a circumstance which is 

 generally attributed to the influence of deforestation.^ 



Let us now consider the case of a stratified soil formed of strata of varying degrees of 

 permeability. This is a case of most frequent and most simple occurrence. After 

 having passed through the permeable soil the water will be arrested by strata that are 

 impermeable, or are so in a lesser degree ; if the surface of contact of the two soils arrives 

 at a point intersected by a valley there will be along the whole line of intersection a 

 "spring level," as it is called. If the strata of the soil are inclined in a certain direction 

 the springs will be more numerous on the slope of the mountain corresponding to that 

 direction. They will rise at points where the line separating the strata of the soil 

 takes a turn, or else at points where this line is cut by a "thalweg" or ravine. It is 

 easy to see that there may be several spring levels along the slopes of the same hill; 

 it is only necessary for this that the strata become less and less permeable. Faults, 

 or a thousand other geological accidents, can alter the regular course of springs. 



Nothing is more variable than the hydrological course of formations which owe their 

 permeability entirely to fissures.^ It generally happens that limestone and sandstone 

 formations which are most cracked at the surface, are quite compact at their bases. 

 Water cannot form continuous sheets in these; it concentrates therefore in pockets 

 and fissures; the overflow no longer takes place in regular lines, as in other soils, but at 

 veritable points of selection. Springs have often a very considerable underground 

 course; they flow from the waters which have filtered through from vast plateaiis; they 



^ The usual direction in the movement of subterranean waters towards a river can 

 be reversed,' when the latter is in flood. Daubree has shown this very clearly in the 

 case of the subterranean sheet of water extending along the course of the Rhine in 

 Alsace. ^ De I^apparent. ' De Lapparent, op. cit., p. 199. 



