274 ^'R MILNE ON THE MID-LOTHIAN AND EAST-LOTHIAN COAL-FIELDS. 



side of a slip, — whether every slip has one which joins it at right angles, or any 

 other angle. On these, however, I will not enter. The data I have collected in 

 regard to them, are too meagi*e to waiTant any conclusion. 



VIII. Before concluding what I have to say regarding the stratified rocks of 

 the district, let me observe, that, in working the coal, Hydrogen gas and Carbonic 

 add gas are met with. 



I am not aware, however, of hydrogen gas having been met with at more 

 than one place, viz. Prestongrange. The workings there are in the immediate 

 neighbourhood of a whinstone dyke, to be afterwards described. About a cen- 

 tury and a half ago, there was coal worked at Wardie ; and Sinclair mentions, 

 that one of the reasons why the working of it was abandoned, was the danger 

 arising from what he calls " wild-fii-e," and which, from his description of its ef- 

 fects, could have been nothing else than hydrogen gas. His words are — " The 

 place where this («. e. the wild-fire) was most known, was in a coal be- west Leith, 

 in a piece of land called Wardy, which, for want of level, and the violence of that 

 fire, the owners were forced to abandon." p. 294. I think that Sinclair's ac- 

 count must be erroneous, for Captain Boswall informs me, that, during his re- 

 cent operations in working and boring for coal, no hydrogen gas was met with. 



The carbonic acid gas occurs in most pits throughout the district. It oc- 

 curs in the gi"eatest quantity in the Roman Camp workings, which are close 

 upon the great body of limestone ; it is also very abundant in the workings of 

 the North Green at Gilmerton, and in the workings at Tranent. It has been sup- 

 posed that this gas is disengaged by the partial disintegration of the subjacent 

 limestone, and that it rises up through the crevices and fissures. But in refe- 

 rence to this opinion, it is proper to observe, that choke-damp occurs in workings 

 of coal far removed from any limestone strata. 



II. UNSTRATIFIED ROCKS. 



I proceed now to an account of the unstratified rocks of the district. 1 . The 

 trap-rocks of the district may be conveniently divided into hills and dykes. 



(1.) There are no kills or amorphous masses of trap within the proper boun- 

 daries of this coal-field. They are all beyond the (yrop of the workable coal-seams 

 and limestones. The only places where these masses of trap occur are the fol- 

 lowing : — Loehend, Edinburgh Castle Rock, Calton Hill, Arthur Seat, Braid Hills, 

 Pentland Hills, Blackrock (near Blackshiels), Barrows (near Giford), Morham, 

 Traprain, and the Garlton Hills. The greenstone and basalt occur at Loehend, 

 Salisbury Craigs, Black-rock, and Barrows. The Braid Hills, Pentland, Garlton 

 Hills, and Traprain Law, are composed oi felspar porphyry. On the Calton Hill 

 and Arthur's Seat are enormous accumulations of trap tufa. 



