86 Dr. H. Franklin Parsons. 



of that formation. At La Roche the geological formation is 

 slate ; the precipitous cliffs characteristic of the limestone are 

 absent, and the general aspect of the country is a succession of 

 deep valleys with steep wooded sides, and occasional outcrops of 

 slaty rock, like North Devon or Montgomeryshire. 



The limestone district of Belgium, like other tracts composed 

 of hard limestone rock, is full of caves, of which one of the most 

 extensive and celebrated, the Grotte de Han, was visited. This 

 cave illustrates in an interesting manner the mode of formation 

 of limestone caverns. Natural caves, I may remind you, are of 

 two kinds, and confined to two classes of situations, viz., sea 

 caves, formed mechanically by the force of the waves on the shore 

 acting upon weak places in the cliffs, and limestone caves, which 

 are produced by the solvent action of water holding carbonic 

 acid in solution, traversing the fissures of the rock. Natural 

 caves in inland districts are only found in limestone, though 

 occasionally in soft sandstones there are extensive series of arti- 

 ficial workings made as places of abode, or for the purposes of 

 getting stone. 



At Han-sur-Lesse the river Lesse, a stream about the size of 

 the Mole at Leatherhead, runs in a U-shaped valley, the centre 

 of the bend being occupied by a hill rising about 300 ft. high 

 above the valley ; this hill is composed of thick bedded carboni- 

 ferous limestone, dipping at an angle of about 45° to tlie north. 

 The river Lesse enters a rocky opening, the Perte de la Lesse, in 

 the side of this hill and disappears, reappearing on the other 

 side of the hill, after a subterranean course of about a kilometre 

 (five-eighths of a mile), measured in a straight line between the 

 points of entrance and exit. The difference in level between the 

 points of entrance and exit of the river, as shown by the map, is 

 ten metres, or thirty-two feet. I was informed that experiments 

 had been made by pouring coloured water into the river at the 

 Perte, and that the colour was not perceived at the exit until 

 twenty-four hours had elapsed ; also that a horse and cart with 

 the driver had been carried into the Perte during a flood, and 

 had never reappeared. These circumstances would seem to show 

 that the river in its underground course must traverse large 

 cavities or reservoirs of water. The valley between the Perte 

 and the Sortie is now devoid of any stream, though it has 

 evidently been excavated by the river at a former period. About 

 a quarter of a mile below the Perte is an opening, or rather a 

 series of openings, in the rock, into which the river must once 

 have run, judging from the remains of an old river channel 

 leading to it, and from the water-worn condition of the rocks 

 around the openings. This was formerly the entrance to the 

 cave. The present entrance for visitors is on the hillside near 

 it, but on a higher level. The passage through the cave takes 



