THE JURA. 
27 
which runs into the Lake of Ter. The southern is 
traversed by the Upper Orbe, which falls into the 
Lake of Joux, and its continuation, the Lake of 
Brenet. Neither of these has any open outlet, but 
the waters escape by an underground passage, and 
reappear above Vallorbes. 
This had long been suspected, but was eventually 
proved in 1893 by M. Picard, who poured in some 
fluorescine, which after the lapse of 30 hours re- 
appeared in the source of the Orbe. 
Again the valley of La Brevine is a synclinal of 
Cretaceous rock, surrounded by Jurassic, and entirely 
closed. The waters escape through several swallow 
boles or “emposieux.” These, however, sometimes 
get choked, and the valley is flooded. 
It has been suggested, as already mentioned, that 
before the sinking of the Rhine valley at Basle, which 
opened a new route to the north, it belonged to the 
river system of the Danube; there is also some 
ground for thinking that at a subsequent period it 
continued its course W.S.W. to the Saone, and that 
the valley of the Doubs below St. Ursanne is in fact 
an old bed of the Rhine. This interesting point, 
however, can only be determined by further evidence. 
The southern limit of the Jura forms the great 
mountain wall, which, from the Lake of Geneva 
1 
