MR MILNE ON THE MID-LOTHIAN AND EAST-LOTHIAN COAL-FIELDS. 259 



Then, as to the position of these several strata in regard to one another, se- 

 veral important remarks occur. 



(1.) There is almost invariably a stratum of fire-clay below a seam of coal. 

 In no instance have I seen or heai'd of coal resting immediately on sandstone or 

 limestone. 



(2.) On the other hand, there are frequent instances of a seam of coal being 

 immediately overlaid by sandstone or by shale. 



(3.) Sometimes there is a seam of clay or shale in the middle of a coal-seam. 

 Occasionally, but very rarely, is there sandstone or limestone in such a position. 



IV. I have alluded to the average, as weU as to the maximum thicknesses 

 of these strata. But it must not be supposed, that every individual stratum 

 maintains, in all parts of the district, a uniform thickness. On the contrary, 

 their thickness varies, — some of them to a very large extent. 



It might lead to very important inferences, could a correct table be constructed 

 so as to shew at one view the thickness of the same individual strata at a great 

 many different points. Such a table would shew not only the maxima and mi- 

 nima of variation, and the rate of variation with reference to space, but, what is 

 much more important, it would shew whether the strata thickened, or whether 

 they thinned away in a particular direction. 



I have attempted to construct such a table, applicable to the coal and lime- 

 stone strata of the East-Lothian and Mid-Lothian coal-field. The great practical 

 utUity of such a table to miners, coal-proprietors and coal-lessees, this is not the 

 place to point out. I refer to it now, for the purpose of shewing the important 

 geological truths which may be derived from such classifications of facts. 



(1.) On examining the table, for the purpose of ascertaining where the coal 

 strata attain a maximum thickness, it will be found, that there is a certain part 

 of the district in which a maximum thickness is attained. This part of the dis- 

 trict embraces Niddiy, Drum, and GUmerton, on its west side ; Prestongrange, 

 Wallyford, and Elphinstone, on its east side ; Cowden and Bryants on its south 

 side. The coals which run through the district now described, are thicker there 

 than they are in any other parts of the district. 



(2.) It win farther be seen from the table, that the part of the district where 

 the coal-seams become thinner, is chiefly to the E. and S. of the places above 

 mentioned ; and that along the E., SE., S., and SW. margin of the coal-field, all 

 the coal strata gradually thin away, so as not only to become unworkable, but to 

 disappear entirely. 



Towards the north of this part of the district, there are not the same means 

 of estimating the diminution of thickness, because the space is so small between 

 it and the sea. That there is a diminution in the thickness of the seams in that 

 direction, there can be no doubt. 



