MR MILNE ON THE MID-LOTHIAN AND EAST-LOTHIAN COAL-FIELDS. 291 



with the fact ; — it is in the upper series of coals, that the sandstone " saddleback" 

 of New Craighall, and the diamond coal (the most irregular of all), occur. 



These views may, at first sight, appear inconsistent with the fact supposed 

 to have been demonstrated by Dr Hibbert, that the limestone of Burdiehouse 

 and the strata contiguous to it, lying at the very bottom of the coal-basin,* 

 were deposited in waters nearly if not entirely fresh. But there is no real in- 

 consistency. Burdiehouse is nearer to the Pentland Hills than any other part 

 of the basin. It was therefore nearer the shore of the ancient sea, than the Gil- 

 merton limestone was ; — and it is not difficult to understand, how there may have 

 been a greater admixture of fresh-water at the former place, than at the latter. 



I remarked, in the first part of this Memoir, that, beneath every seam of coal, 

 there is invariably a bed, more or less thick, of clay. This is perfectly consistent 

 with the notion, that the coal-seams owe their origin to accumulations of vegeta- 

 bles uprooted and carried off, having attached to them a quantity of the soil on 

 which they grew. 



But whatever be the way in which the vegetables composing the coal strata 

 have been brought, it is exceedingly probable, from the internal structure and 

 organization of coal, that, after its deposition, the vegetable matter has been in- 

 fluenced by chemical affinities ; — and this circumstance may have to a certain ex- 

 tent assisted, in creating a uniformity of thickness in the different strata. In one 

 and the same seam of coal, it often, nay, it most generally happens, that there 

 exist several different kinds of coal. For example, in the " Great Seam," there is 

 (1), the rhomboidal, cubical, or cherry coal ; there is (2), the splint or slaty coal ; 

 and there is (3), the conchoidal or parrot coal ; — and specimens of these several 

 varieties may be got in the same hand specimen. Now, aU these possess more 

 or less a crystalline structure ; and, what is more, each of them has a peculiar 

 crystalhne structure of its own, each being separated from the other by a dis- 

 tinct line of demarcation, called, in the language of colliers, a " parting." 



The peculiarity of crystalline structure which characterizes each kind of coal, 

 depends (as was explained in the first part of this Memoir) on certain joints or 

 fissures which intersect the coal, and which intersect the coal at different angles 

 in each variety. 



There must, of course, have been some important difference in the constitu- 

 tion and condition of the vegetable matter which produced these different varie- 

 ties of coal, and gave to each a peculiar crystalline structure. Accordingly, it 

 appears, from the analysis of Dr Thomson, that each kind of coal has a different 

 organization. His analysis shewed, that the following were the proportions of 

 elementary substances in the different kinds of coal. 



* See Appendix E. 



