MR MILNE ON THE MID-LOTHIAN AND EAST-LOTHIAN COAL-FIELDS. 265 



serve a pretty uniform thickness, do occasionally vary in thickness in an ex- 

 traordinary manner. At New Craighall, in the Beefie coal worked there, a band 

 or stratmn of clay occurs 1 foot thick. At Sheriffhall (about two miles distant) 

 this band of clay ihickens to 18 feet, thereby, of course, dividing the Beefie 

 coal into two separate seams. 



In the example now mentioned, it seems probable that the variation in thick- 

 ness is characteristic of the original formation of the stratum, and arose from a 

 larger quantity of sediment being deposited in one part of the district than in 

 another. The example next to be given seems to indicate a different cause for 

 the variation in thickness, — and one which operated subsequently to the deposi- 

 tion of the stratum. 



At Bryants, the " Coal Patie " is generally 3 feet thick, and is overlaid first 

 by a bed of shale 2^ feet thick, and then by a stratum of coarse sandstone. At 

 one spot, however, the shale disappears, and about a foot of the coal from its up- 

 per surface also disappears. In this way an expanded hollow or trough is pro- 

 duced, which is filled up by a protuberance of the sandstone stratum. 



A similar example occurs in the workings about a quarter of a mile to the 

 SE. of East Houses. There is or was formerly a small " rough coal " worked 

 there, about 3 feet thick, covered, as in the former case, first by shale, and 

 next by a soft yellowish sandstone. At one place the shale disappears, and the 

 coal is diminished in thickness to 2^ feet, the trough being occupied by the sand- 

 stone. 



V. The next subject to which I would advert is the form or outline which a 

 section of the whole deposit presents in superficial extent, as well as in depth. 



I have, with the view of making my explanation more intelligible, drawn 

 some figures, representing along particular lines (crossing the entire coal-field), 

 the form of its surface, and the supposed undulations of the several strata. 



Before pointing out the details of these sections, let me mention how they 

 have been formed, so that the Society may judge what degree of weight, or whe- 

 ther any weight, is to be attached to them. 



I traced out, in the first instance, on a map of the district, the croppings of the 

 principal coal and limestone strata, and laid down the course of all the dykes and 

 slips intersecting these strata, of which I could get information. In this way I 

 got the distances between the coal-seams at the surface, and knowing the angle at 

 which they dipped, it was easy to represent the strata in vertical sections, with their 

 different inclinations and undulations. I have also shewn, wherever these lines 

 of section are intersected by slips or dykes, on which side the strata are downcast 

 or upcast (as it is termed) ; — and I have endeavoured to give a correct outline of 

 the surface of the country along these lines of section, with reference to its eleva- 

 tion above the sea. In constructing these figures, I have made the vertical sec- 



VOL. XIV. PART I. L 1 



