L. C. Miall — Formation of Swallow-holes. 513 



Boulder-clay may be entirely different from that referred to by an- 

 other. This, too, may be illustrated by Mr. Harmer's section of the 

 Eastern Glacial and Post-glacial beds, where, as you saw, and in 

 which I am happy to conjB.rm him from personal observations in. 

 Holdemess,^ as clearly distinct a series of beds occurs, as that I have 

 just described. I differ from him and Messrs. Eome and Wood, in con- 

 sidering the beds above the G-rey Chalk clay Glacial, there being on 

 the chalk clay as distinct a denudation and unconformability as in 

 the bed F, in the Llandudno sections. Possibly our variance may 

 arise from want of a more distinct definition of the meaning of the 

 term " Glacial." I agree with Mr. Hughes in the opinion he ex- 

 pressed yesterday, that the time is not yet come, when the beds of 

 the eastern and western Drift series should be correlated ; but I think 

 I already see evidences of a greater possibility of correlating them, 

 than some of the workers in the eastern districts (whose untiring 

 zeal in the elucidation of those beds is above all praise) are willing 

 to admit. 



Note. — In the paper on Holdemess already referred to, I insisted 

 on the " Grey Clay," as the only " true Glacial Boulder-clay," and 

 which my subsequent observations have only served to confirm. 



General Section of Drift in neigliboiirhood of Llandudno. 



A Sand-dunes. Thickness very unequal. 



B Blue-black clay without pebbles, 1 foot. 



G Sands and gravels; about 50 feet. 



D Red clay, greatest development about 20 feet. 



E Sands and gravels, greatest development about 20 feet. 



E2 Stratified grey clay, 20 feet. 



F Boulder-clay, greatest development 150 feet. 



G Mountain Limestone Eubble, 3 to 5 feet. 



H Bedded Mountain Limestone. 



Y. — On the Formation of Swallow-hoebs, ok Pits with ver- 

 tical Sides, in Mountain-limestone.'' 

 By L. C. Miall, Esq. 



THE geologist who rambles through the Mountain-limestone dis- 

 trict of Yorkshire is sure to have his attention arrested by the 

 numerous "swallow-holes," or pits, in the rock, into which rills, 

 streams, or even rivers precipitate themselves ; and if he can bring 

 his mind to consider the subject sufficiently, without dismissing it at 



1 See Drift sections of the Balderness Coast, read before Liverpool Geological- 

 Society, Dec. 11, 1866, and published in their Proceedings. 

 3 Eead in Section C. of the British Association, Liverpool, September, 1870, 



