240 Dr Hibbert on the Limestone of Burdiehouse 



into account, that this increased dip might have been caused by 

 some renewed, yet very slow elevation of the Hawthornden sys- 

 tem of strata, by which a sort of lateral pressure had been in- 

 duced among the contiguous strata of the Burdiehouse system, 

 sufficient to increase their inclination from 23° to 56° or more. 



Having at length briefly described the systems of strata to be 

 found in the great coal-field of Mid-Lothian, as indicated by 

 fractured areas, I shall now confine my attention to the connected 

 ones of Gracemount and Burdiehouse. 



SECTION III.— THE GRACEMOUNT SYSTEM OF STRATA. 



Before describing the beds to which the limestone of Burdie- 

 house belongs, it will be necessary to previously notice the strata 

 lying to the west of, and separated from, the Burdiehouse system 

 by a fine of fissure. The Gracemount system occupies a place 

 intermediate to this fault and the trap of the Pentlands. 



A sandstone, which perhaps holds the lowest place in the car- 

 boniferous system of the great Lowland valley of Scotland, is com- 

 posed of fine, yet very uncrystalline, grains of siliceous matter ; it 

 contains little mica ; it is of a deep red colour, owing to the iron 

 which is diffused through it ; and it is of a soft consistence. This, 

 the most deep-seated bed, rises to day in a few places only, and, 

 if it is to be traced in the group of strata to the west of the lime- 

 stone of Burdiehouse, we must perhaps look for it among the 

 upheavings which occur in the immediate vicinity of the range of 

 trap rocks, extending from the chain of the Pentlands, with some 

 little interruption, to Arthur's Seat. Most probably this sand- 

 stone may be proved to be wholly, or in part, of marine origin, 

 — a suspicion which I have formed upon certain organic remains 

 apparently referable to this deposit. 



But quitting the consideration of a sandstone, the existence 

 of which in the Gracemount system is at best dubious, owing 

 to the rocks being here much concealed, I shall proceed with a de- 

 scription of what is actually known. 



