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



Having thus stated the aggregate thickness of each class of stratified rock, 

 I may mention the number of sti-ata composing respectively these different classes, 

 as well as the average thickness of each particular stratum. 



There are between fifty and sixty coal-seams exceeding a foot in thickness. 

 None of them are more than 13 feet thick. The average thickness is about 3^ feet. 



I cannot, for the reasons already noticed, state the actual number of the 

 other strata ; but in that part of the coal basin best known, which as I have said 

 includes about one-half of it, the following are the respective numbers : 



Sandstone strata above 4 feet thick . . 47 



Shale 3 ... . . 62 



Lime 2i ... . . 9 



Clay 3 ... . . 8 



The maximum thickness which the sandstone strata reach appears to be 

 about 200 feet ; that of the shale 180 feet ; that of the limestone 40 feet ; and that 

 of the clay 28 feet. 



11. I come next to notice the superficial extent of the strata. I here allude 

 not merely to the occurrence of the same kind of rock over a given tract of 

 country, but to the extension of individual strata. One of the strata of coal 

 worked in the district, is known by the name of the " Ch^eat Seam.'" It may be 

 seen in a quarry at the east end of Portobello. This seam was foiinerly worked 

 at the following places, viz. Brunstain, Niddry, Edmonstone, Woolmet, Drum, and 

 Gilmerton. It is (on the same line of bearing) still worked at Loanhead and at 

 Dryden, places that are eight or ten miles from Joppa, in a SW. direction. But 

 the same seam is also worked along the ridge mentioned at the outset of this 

 memoir, viz. the ridge which runs from the Roman Camp to Ti-anent, and it is 

 worked on both sides of that ridge. On the NW., or Esk side of the ridge, it is 

 now worked at the following places ; Arniston, Bryants, Prestongrange, and at 

 the pit recently opened by the Duke of Buccleuch at Cowden. Formerly it was 

 worked at several intermediate points, as Carherry and Wallifm^d. On the SE. 

 or Tyne side of the ridge, this " gi*eat seam" is or has been worked at Vogrie^ 

 Edgefield, Oxenford, EliJhinstone, Birsley, Tranent, and Prestonpans. Along this 

 ridge, therefore, it runs on both sides for twelve or thirteen miles. But the same 

 seam is Imown even beyond this line, and on the extreme eastern limits of the 

 coal-field, viz. at the Bents (E. of Cockenzie), at ♦S'^ Germains, Blindwalls, and 

 Cinderhall. 



I may add, for the sake of gi\'ing a more distinct idea of this matter, though 

 it is anticipating a little what I will after^vards more fully explain, that this 

 coal stratum, on the west side of the basin (as at Gilmerton, Niddry, &c.) dips 

 down rapidly to the SE., — becomes flat in the middle of the Esk valley, — and 

 rises up to the Roman Camp and Carberry Ridge ; — it then mantles over that 



