THE SUBTERRANEAN RIVER MIDROI. ' 257 



infinitely small ones like the microbes pass. Although the thick- 

 ness of the calcareous mass here exceeds eight hundred feet, the 

 filter is not homogeneous, but is fissured ; and through these 

 faults, these cracks, in which the circulation is yet slow enough 

 for the water, coming in muddy at the level of the plateau, to 

 issue at the spring with admirable purity, the microbes continue 

 to percolate. It is true that the microbes I have found are not 

 pathogenic, but the importance of the studies can nevertheless be 

 comprehended. If common microbes, brought in by the waters 

 that fall on the plateau, can be found eight hundred feet below it, 

 there is nothing to prevent noxious microbes those, for example, 

 of typhoid fever, diphtheria, or cholera which may live in the 

 water from being found there. There is in this a very interesting 

 problem of hygiene and public prophylaxis. We should, then, be 

 suspicious of these beautiful crystalline springs when there are 

 epidemics on the plateau from which they come. They may con- 

 tain micro-organisms some indifferent, others dangerous. The 

 microbes which I found belong to the genus micrococcus. Two 

 of them (Micrococcus aurentiacus and M. citreus) developed in 

 fine colonies of orange and citron-yellow colors; a third (M. 

 aquatilis) gave no coloring matter. I obtained microbic colonies 

 after the twentieth hour. 



What, in short, is this underground river Midro'i (Fig. 2) ? A 

 large fissure through which flow the waters drawn from the pla- 

 teau by the avens, orifices, and cavities of every kind which make 

 an enormous sponge of the mass of the Causses. In past centu- 

 ries, when the mass of water that fell on the Causses was consider- 

 able, Midroi acted regularly and gradually enlarged the fissure ; 

 but now it acts only intermittently. Its vent then affords it 

 sufficient outlet. Let me speak of this vent, the fine spring of 

 Rochemale, which issues from the rock a little more than one 

 hundred yards west of the orifice of Midro'i. By this narrow 

 fault, the communication of which with Midroi is highly prob- 

 able, although it is not demonstrated, two hundred and twenty 

 thousand litres of water escape every hour. In case the supply is 

 doubled after great rains, the water, which can not escape by 

 Rochemale in so large a quantity and in the same lapse of time, 

 rises and fills all the meshes of the sponge. If the supply is in- 

 creased again, the water flows into the river Midroi, which then 

 comes into operation and the level of which may rise several 

 yards, as is shown by the traces of wash-marks left by recent in- 

 undations on the walls of the river and which are shown in our 

 photograph. There exists, in effect, in the very heart of the 

 Causses, a considerable and eminently variable reservoir of water ; 

 it is a real lake, and through the thousand fissures, through all 

 the meshes of this interior region, flow the waters of the plateau, 



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