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



wliere the surface around has not been covered with alluvium or 

 drift to a considerable depth. On a bare limestone plateau, such as 

 the edge of Malham Cove, and also in non-calcareous strata, the joints 

 weather into fissures, often broad and deep, and we may find 

 flutings of various sizes formed as channels for trickling water, but 

 the pits do not assume the same shape, nor are the flutings vertical 

 and sharp-edged. The similarity of these two kinds of excavations 

 is of a very general kind ; and when we come to details it is soon 

 clear that the Buttertubs, for instance, are due to causes less simple 

 than the mere abrading or dissolving action of running water. Con- 

 stant drip from a tolerable height upon the same place, and the 

 friction of loose pebbles in the receiving basin, appear to be neces- 

 sary to the production of their characteristic features, and these 

 agents of waste are traceable to the thick alluvial covering. Its 

 absorbent character enables it to act as a sponge, retaining the rain- 

 fall, and distributing it slowly and regularly. The overhang of the 

 turf gives the drops an uninterrupted fall, and the sand and pebbles 

 which fall from it help to grind the concave surface below. In some 

 cases I have noticed that on the sides where drift still lies thick the 

 flutings are sharp and still forming, while in another place, where 

 the edge of the pit has been long bared, we have a face of rock 

 weathered irregularly, and losing its grooved appearance till it. 

 passes into a plain limestone escarpment. 



We must not forget the important influence exerted by those 

 vertical divisions of the rock, formed subsequently to deposition, 

 which are known as joints. In mountain limestone these are com- 

 monly conspicuous, though not close-set, except in a thin-bedded 

 stratum, and they are more regular than in most sandstones. Lime- 

 stone is rarely false-bedded, and its homogeneous character favours 

 the production by expansion and contraction of rectangular dividing 

 planes. To these crevices rain and air get access, and enlarge the 

 cracks into fissures. The edge of many a limestone cliff is thus 

 broken up into cubical blocks, and this is all when the sole weather- 

 ing agent is equally-distributed rainfall. But when the drip from 

 a spongy mass of alluvium and turf forms the little cascades de- 

 scribed above, we have a tendency to produce new and irregular 

 forms by the repeated excavation of semicylindrical grooves in 

 various parts of the vertical faces of rock. The pit enlarges uncer- 

 tainly, but generally tends to a circular form, leaving now and then 

 isolated pillars, which may rise from a ledge in the pit or from the 

 floor itself, according as they have or have not continued to be acted 

 upon in the same way as the surrounding masses. A small pillar 

 from which the cap of alluviun has been swept, while the swallow- 

 hole has not ceased to grow deeper, shows, by its attachment to the 

 adjacent rock at a greater or less depth, when its base has been 

 removed from the general waste. What happens after this depends 

 upon the accidents of the place. The swallow-hole may enlarge 

 indefinitely, following the course of the principal fissures or extend- 

 ing itself independently, and becoming more or less circular. In 

 other cases its enlargement is arrested by the fall of the alluvium, or 



