L. C. Miall — Formation of Swallow-holes. 515 



side. The floors of most of the pits are covered with debris and 

 rounded pebbles (chiefly of sandstone), which appear to be derived 

 from the drift, and, ultimately, from a bed of grit (the ten-fathom 

 grit), about 150 feet above the main limestone. The water which 

 enters the holes penetrates the limestone horizontally, and escapes 

 beneath it over a ledge of grit. 



We have, therefore, in these Buttertubs many features worthy of 

 notice, and unusual opportunities of investigation. I have en- 

 deavoured to satisfy myself as to their origin, and shall now ofi'er 

 some remarks on the principal difficulties which suggest themselves. 

 It will be convenient to regard them as the type of that kind of 

 swallow-hole whose mode of formation I shall first describe. It 

 will afterwards appear that there is another and quite distinct sort 

 of swallow-hole, whose walls are only fluted in a few instances and 

 to a trifling extent. They seem to be due to a different set of causes 

 altogether. 



On searching for some indication of the way in which the Butter- 

 tubs had been formed, I was led to examine a miniature swallow- 

 hole, near the road. Here is a little grotto overhung by turf, which 

 mats together the clay and pebbles of the drift. From this spongy 

 roof water drips incessantly upon a ledge of rock two feet below. It 

 has excavated a concave surface or cup, which contains a number of 

 small pebbles smoothed by friction. These pebbles are not of lime- 

 stone, but of sandstone, and probably come from the ten-fathom grit, 

 which makes a low escarpment higher up on the side of the glen. 

 We hear the rattle of the pebbles amidst the plash of the falling 

 water, and recognize in the sound evidence of continual abrasion ; 

 slow indeed, but more rapid than the action of water upon a uniform 

 surface. The ledge is being gradually lowered, and as the exca- 

 vation proceeds it leaves a scar upon the face of the limestone wall, 

 semicylindrical and with sharp edges, such as would be cut by a 

 gouge. Close by are a score of similar grooves, all formed in the 

 same way ; some long completed, but still sharp and clear, others 

 now in process of excavation. Elsewhere we see the same force at 

 work on a larger scale, but a little disguised by accidents of situation 

 or of rock-form, and we find in these cascades, nourished by the 

 accumulated moisture of a retentive soil, and aided in their work by 

 the pebbles which have fallen into the basin, a possible source of the 

 flutings which cover the vertical faces of the pits and of the isolated 

 pillars. 



Further observation justifies the conclusion that these large and 

 deep cavities are indeed thus produced, but questions and difficulties 

 arise which can only be removed by an examination of the details in 

 the closest manner. The regular supply of dripping water from the 

 same point, the connexion of the pits with joints or fissures, and 

 the association of some of their most remarkable features with one 

 kind of rock, and one only, — these all require elucidation before we 

 can fully understand how so insignificant a cause can lead to such 

 conspicuous results. 



I do not recollect an example of a swallow-hole of this kind 



