BY HTGH MTLLEE. 341 



of a house. Each expansion thrust the metal a little downwards ; 

 each contraction, having to pull up against weight, failed to draw 

 it up as before ; and the result was a downward gravitation. 

 I have long suspected that the masses of rock in question were 

 slowly travelling downwards from the same cause, but should 

 scarcely have ventured to affirm the fact, but for the experiences 

 of Sir J. S. Hooker among the Himalayas, and Sir "Wyville 

 Thomson in the Madeira Islands. The causes that set ''stone 

 rivers" in motion in these places, must be at work in a minor 

 degree everywhere, and it need not be doubted that the parti- 

 tion of escarpments is aided by their means. 



3. Swallow -holes. — It is common to find that when the roof of 

 a colliery level, or underground gallery, gives way near the en- 

 trance, the surface of the ground, without breaking into a con- 

 tinuous trench, sinks down here and there into rounded pits and 

 dimples. Limestone joints widened into fissures by aqueous 

 solution may be likened to a network of levels, of which denu- 

 dation is gradually uncovering successive portions. And when 

 the roof of clay or shale becomes thinned until it can no longer 

 support itself, it breaks through, probably at points where the 

 fissure is wider, or a stone or nodule concentrates weight, — and 

 pits the surface. Such is the origin of the swallow-holes that 

 border many of the limestone outcrops in this district. 



The greater number of swallow-holes intercept drainage and 

 become sinks, but this, which is apt to be mistaken for their pri- 

 mary feature, is really a secondary and accidental one. It is the 

 ground that is swallowed in the first instance, and as a subsequent 

 act the hollow may become the centre of a little drainage basin. 

 Without a covering of loose materials, such as shale, clay, or peat, 

 the water descends, so far as I have seen, through simply a 

 widened rift. The largest swallow-holes, on the other hand, are 

 those opened through the thickest covers. 



In their earlier stages swallow-holes arc simple round bowls. 

 When the swallowing is complete, the bowl becomes perforated, 

 and of the form of the receiving part of a common filter. The lip 

 is generally channelled by the little stream that enters. In size, 



