MR MILNE ON THE MID-LOTHIAN AND EAST-LOTHIAN COAL-FIELDS. 333 



sent sea. This would lead to the supposition, of there having been an elevation 

 of the land at a period immediately antecedent to the deposit now referred to. 



I have alluded to the remains, both vegetable and animal, imbedded in the 

 clay which form the lower part of this deposit. All the vegetable remains are 

 terrestrial, and are such as to indicate the existence of a river or rivers of con- 

 siderable size. The animal remains are marine ; — the shells, such as are usually 

 found in great estuaries, where there is a commixture of salt and fresh water. 

 It seems probable, therefore, from a consideration exclusively of these remains, 

 that during this particular epoch, when clay and sand were deposited, there exist- 

 ed a bay or arm of the sea into which ft-esh water flowed. But as the waters 

 then stood at least 700 feet higher than they now do, that arm of the sea must 

 have stretched across towards Glasgow, in which case Arthur Seat, and most 

 of the Pentland hills, were islands, as Inchkeith and Inchgarvie now are. Into 

 this sea, would of course be poured immense quantities of muddy sediment 

 derived from the boulder-clay, and which would be deposited in all the hollows 

 of the boulder and stony clay. But at any given place, it would not be at 

 every moment deposited in exactly the same quantity. The tides would in this 

 respect have a considerable influence. As the largest supplies of the muddy sedi- 

 ment would be afforded by rivers, the deposition would be suspended, or at all 

 events diminished, by every influx of the tide — when probably a slight sprinkling 

 of sand would be thrown upon the muddy deposit which took place during the 

 ebb-tide. In this way, we can understand how the laminae visible in the Porto- 

 beUo brick-clay were formed ; — and if this inference be correct, then each layer 

 of clay would denote the period of one tide or half a day. As each layer is, on 

 an average, about one-sixth of an inch thick, 120 feet (which is supposed to be 

 the thickness of the flne clay at Portobello) would denote a period of twelve years 

 as the interval of its deposition. 



If from any cause, the waters ceased to be supplied with muddy sediment, 

 then sand alone would be deposited. It would appear, that for a certain period 

 muddy sediment had been supplied in this district, — after which sand, the natural 

 product of oceanic waters, was accumulated in large banks over the clay. The pro- 

 blem then is, to account for a supply of clay or mud by the rivers in unusual 

 quantity, and for a certain period only. Perhaps a solution of the problem may 

 be found in the elevation of the district which took place immediately before this 

 period. The bottom of the sea as it became exposed to the united influence of 

 rivers and rains, being unprotected by a vegetable covering, would afford an abun- 

 dant supply of muddy sediment, and would continue to afford this supply until 

 vegetation had consolidated the soil, and the rivers had acquired for themselves 

 permanent channels. 



Notwithstanding these extraordinary physical revolutions, and the fact that 

 the sea was at least 700 feet higher than it now is, it would seem that this district 



