1856.] HARKNESS — LOWEST ROCKS OF ESKDALE. 241 



repeated several times both on the north and south side, — and this 

 is the aspect which the deposits of a shaly nature present, which is 

 of such a character as would result from the violent twisting of them 

 and their associated strata. Many of the shaly beds in Eskdale are 

 rarely capable of being divided along the laminae of bedding, but 

 have a broken-up appearance, the small fragments having a rhom- 

 boidal form ; and at one locality in this portion of Dumfriesshire, 

 namely Boykin Crag in the parish of Westerkirk, we have these 

 shaly deposits so far changed (the result of pressure in consequence 

 of oblique curving of the deposits) that the argillaceous beds here 

 are possessed of a decided slaty cleavage, this being the only locality 

 among the Silurians of the South of Scotland where this form of 

 structure occurs, so far as I am aware of. So rare is this form of 

 structure in the mountainous districts of the South of Scotland, that 

 Prof. Sedgwick * cites the district as affording proof against cleavage 

 resulting from lateral pressure, — an opinion in which I was disposed 

 to agree ; but more minute examination of the country, and a more 

 enlarged experience of the nature and features of this structure, as it 

 is manifested in the Devonians of the South-west of Ireland, have 

 induced me to adopt an opposite conclusion. 



The great mass of the strata which make up the Silurians of this 

 part of Scotland are of an arenaceous nature ; a composition which 

 is not so susceptible of being impressed with cleavage as argillaceous 

 deposits ; and they have not been subjected to the same amount of 

 flexuring as those small areas which are more intimately connected 

 with the axis, and which it is the object of this communication to 

 describe ; therefore it is in these latter, which abound in argillaceous 

 beds, that we have that rearrangement of particles which seems to 

 result from lateral pressure in consequence of the oblique flexuring 

 of strata. 



The deposits which compose the beds lying on the bottom-rocks 

 of the South of Scotland present some circumstances which afford 

 considerable insight into the physical conditions under which these 

 were formed. The alternations of thin-bedded sandstones and shales 

 indicate that these strata resulted from comparatively shallow water ; 

 and the abundance of ripple-markings corroborates this inference. 

 In one locality, on the south side of the axis, at Binks in Roxburgh- 

 shire, about three miles N.E. from Mosspaul Inn, the ripple-marked 

 surfaces of the sandstones are well seen, as well as the alternations 

 of the thin argillaceous and arenaceous beds. Here also we meet 

 with evidence of another character, which, considering the extremely 

 low position of these sedimentary rocks, is of an important nature. 



Some of the thin beds forming the deposits in this locality con- 

 sist of alternating layers of very fine sedimentary matter, associated 

 with coarser layers ; deposits which have originally been fine and 

 coarse mud. On the faces of the beds which are composed of the 

 former, in one instance, we have distinct traces of desiccation-cracks, 



* Brit. Palaeoz. Fossils, page xxxvi, note^ 



