518 L. C. Miall — Formation of Swallow-holes. 



The kind of swallow-hole which I have just described is, in all 

 important respects, comparable to the moulins of a glacier. We have 

 in both the conditions of homogeneity of rock or ice, of many fissures, 

 of a ready escape of water through or beneath the excavated mass, 

 and of an equable supply of water over a considerable surface. 



We come next to the consideration of the second class of swallow- 

 holes, whose existence is due to subsidence of an undermined crust, 

 and not to direct excavation. 



Geologists are familiar with the long caverns of mountain lime- 

 stone districts. In the majority of cases they are fissures enlarged 

 by the running water which passes along them. Ingleborough Cave 

 is upwards of 700 yards in length, and appears to traverse part of 

 the course of two or three parallel fissures, the intervening rock 

 being tunnelled through several times. Many instances occur of 

 subsidence of a portion of the roof of such a cavern, and it is not un- 

 common to find several openings into the same subterranean channel. 

 In Hellen Pot, near Horton, in Eibblesdale, there are at least three 

 such shafts. When we find a long, irregularly arched, passage, some- 

 times opening into lofty chambers, and presenting at intervals large 

 apertures, beneath which the floor is encumbered with great blocks 

 of limestone, we naturally conclude that the cavern was first formed 

 by a stream of water, and that weak parts of the roof have subse- 

 quently fallen in. This I believe to be the true interpretation. 

 The sides of such chasms are only rarely and incidentally fluted ; the 

 aperture does not ramify irregularly along the course of the fissures ; 

 there are no isolated pillars. Many of them have no drip of water 

 from the surface ; others are woin to a slope or a spout on one side 

 by the entrance of a mountain stream. Near Settle there is a slight 

 depression in the ground at a place called Eobin Hood's Mill. Here 

 a rumbling noise is continually heard which resembles the rush of 

 water underground. I imagine that at this spot is situated the ex- 

 panded part of some hidden water-course, which may be sooner or 

 later converted into a shaft by yielding of the roof. 



Swallow-holes of this second species frequently occur in a line 

 sometimes in a ring round a hill-side.^ They are most abundant in 

 the lower scar limestone, rarely occurring in. the thinner limestones 

 of the Yoredale series. The formation of the long caverns is favoured 

 by the great horizontal partings which mark the scar limestone. In 

 the regularity and great extent of these partings of the thick-bedded 



Chalk beneath Norwich, which had been tapped by the deep-main sewers lately con- 

 structed, and which had proved almost insurmountable obstacles to the completion of 

 the works. He also mentioned the case of a sudden sinking of land at Lexham, 

 Norfolk, upon the farm of Henry Childs, Esq., leaving a deep circular depression 

 in the field. A ploughman, plough, and pair of horses at work in the field were 

 carried down with the land, but were providentially rescued, without harm, from a 

 depth of twenty feet or more. Such pits, he remarked, were evidently the result of 

 the falling in of the roof of one of these deep-flowing rivers in the Chalk beneath. 



' Prof. Phillips mentions a glen formed by a line of ancient subterranean caverns. 

 The subsidence of the roof appears to have originally determined the direction of the 

 watercourse. See paper on Formation of Valleys near Kirby Lonsdale, British 

 Association Reports, 1864. 



