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



ver}'^ nature and situation of the sandstone strata, becomes still more obvious, on 

 considering some of the facts mentioned in the first part of this memoir. It was 

 shewn by statistical details, that, of all the strata, none is so irregular or variable 

 in thickness as sandstone. This is at once explained by the greater weight and 

 specific gravity of its ingredients, which causes it to be deposited more quickly, 

 than fine muddy sediment out of which the shales and limestones were formed. 

 The latter can remain suspended for a longer period, and in fact wiU not sink to 

 the bottom in any considerable quantity till the water becomes comparatively 

 tranquil. In this way there is time allowed for the sediment being equally dis- 

 tributed through the aqueous medium before it reaches the bottom ; so that, after 

 it does reach the bottom, it forms beds or layers of tolerably uniform thickness. 

 But the case is quite otherwise with siliceous sediment. It is transported but a 

 short way before it faUs to the bottom. There is no time for it therefore to be 

 diffused equally over the district ; and the consequence wiU be, that the beds or 

 deposits of sandstone will, generally speaking, be of very variable and irregular 

 thickness. Whilst, on this subject, I may be permitted to refer to the examples 

 given in the first part of this memoir, of the sudden variations in the thickness 

 of sandstone strata, and in particular to the account there given of that remark- 

 able " Saddle-back," as the coUiers term it, which occurs in the upper part of 

 the coal-basin. It is a sandstone rock, which lies over a particular coal-seam. 

 I can compare it to nothing except a sand-bank, such as is formed in our exist- 

 ing seas. It is at its base about 120 fathoms in width, and it has been traced 

 running in a S.SE. direction for about three miles. It is of a semicircular form, 

 the lower part or base of it being perfectly flat. The top is about 10 fathoms 

 from the base. It is quite impossible to look at the position of this sandstone 

 bed, and see the manner in which the various strata of shale, sandstone, and 

 coal, come up to the sides of it, and then rise over it, diminished in thickness, 

 but not fractured or deranged, without being convinced, Jirst, that the sandstone 

 rock has been formed at the bottom of an aqueous medium agitated by currents ; 

 and, second, that these other strata had been afterwards deposited in the same 

 medium, when m a state of comparative tranquillity. 



So far with regard to the manner in which these several strata were formed ; 

 and the reason why the sandstones should be more variable in their thickness 

 than the other kinds of rock. But after these strata were formed, they would 

 be liable to be worn down and occasionally hollowed out by the agency of cur- 

 rents ; and, therefore, if the above theory as to the mode of their formation be 

 true, we ought to find in the rocks of this district examples of such erosive ac- 

 tion. Tills inference is verified by the fact ; for, it may be remembered that, in 

 the first part of this memoir, several examples were given of hollows in the strata, 

 which were shewn to be fiUed or occupied by the particular stratum lying over 



