MR MILNE ON THE MID-LOTHIAN AND EAST-LOTHIAN COAL-FIELDS. 287 



heat would there have more effect on it, — the other reason is, that, supposing the 

 greywacke hills to have been elevated by subterranean heat, the water along 

 their sides would be warmed, not only from being in contact with them, but also 

 from being near the vents and fissures communicating with the source of heat. 

 On this principle, the fact that the limestone stratum lying under the North 

 Greens coal, is eight or ten times thicker near the Lammermuir Hills than at 

 Duddingston, may be explained. But the theory now suggested may be applied, 

 not merely to explain the particular phenomena here adverted to. It may serve 

 to explain the deposition of limestone strata in all situations. We see, that 

 during the period when the strata which compose this particular coal-field were 

 being formed, there were altogether five or six deposits of hme. May this not 

 have been caused by heat being communicated, from subterranean sources, to the 

 aqueous medium holding the carbonate of lime in solution ? At each accession 

 of heat, there would be precipitated a stratum of lime, extending more or less 

 over the whole district, in proportion to the general diffusion of the heat. This 

 agent would be more efficient in its operations at first, i. e. before any great num- 

 ber of strata had accumulated ; for, in proportion to that accumulation would be 

 the distance of the aqueous medium from the subterranean heat, and the means of 

 intercepting it. Accordingly, we find, that, in this district, the thickest deposits 

 of lime are in the lowest parts of the basin, and the thinnest above ; and further, 

 that, in the upper half of the series, no lime strata exist at all. 



In saying that, in the upper half of the series, there are no lime-rocks, we do 

 not mean to say that the strata are entirely devoid of all calcareous matter. It 

 is found that the sandstones and shales are in all parts of the series more or less 

 impregnated with carbonate of lime. For example, in the Craigleith sandstone, 

 a small proportion of this substance exists. It is a remarkable fact, that, in the 

 fossil trees imbedded in the Craigleith sandstone, carbonate of lime should form 

 more than one-half of their substance, and that oxide of iron and magnesia should 

 exist also in a considerable proportion, whilst hardly any silex is to be found in 

 them. It is very obvious, from these facts, that the sandy sediment had been 

 deposited at the bottom of an estuary which held in chemical solution a large 

 proportion of carbonate of lime, magnesia, and iron. The liquid containing these 

 substances would soon make its way into the interior of the trees, though the 

 grains of sand could not ; so that, when the water evaporated, these carbonates 

 would be left in the substance of the fossil. 



The view above suggested, for explaining the greater thickness of the lime- 

 stone strata near the Lammermuir range, might be employed to explain the large 

 proportion of iron in the old red sandstone formation : for, if bicarbonate of iron 

 was held in solution by the waters which covered the district, the higher tempe- 

 rature of the water along the flanks of the hills would drive off a part of the car- 



