Great Valley of the Scottish Lowlands. 109 



stone noticed by Mr Murchison at Pontesbury, and other places 

 in the Shropshire coal district ; for the opinion expressed by M. 

 Agassiz, relative to the ichthyolites which have been found in 

 the Scottish coal-fields, has shewn that neither the Burdiehouse 

 limestone, nor any of the other beds of the carboniferous group 

 in this country, with which we are at present acquainted, have 

 any claim to be so considered ; nevertheless, there are peculiari- 

 ties in the character of the limestone at Burdiehouse, both in re- 

 gard to its organic contents, and to its position in the series, 

 which render it an object of the highest geological interest : and 

 the discovery by Dr Hibbert, of a new species of sauroid fish, the 

 Megalichthys, in that limestone, is one of the most important 

 that has been made in the present times. 



In Scotland, the greater part of the secondary formations, in- 

 cluding the coal-measures, have been deposited in that extensive 

 district which now forms the great valley of the Scottish Low- 

 lands, and separates the primary country, of which the northern- 

 most extremity of Great Britain is chiefly composed, from the 

 transition chain of the southern border (at that time probably 

 partly submarine), which may be geologically considered as the 

 South Highlands. 



According to Williams*, a fine drawn from the mouth of the 

 Tay, passing through Stirling to the northern extremity of the 

 Isle of Arran, and another nearly parallel to it from St Abb's 

 Head on the east coast, to Girvan on the west, will include be- 

 tween them the whole of the coal-fields of Scotland, with the ex- 

 ception of the insulated coal basins on the Nith and the Esk in 

 Dumfriesshire, and the seams of coal that have been worked in 

 Roxburghshire and on the coast of Berwickshire. The coal beds 

 of Brora, and those that have been met with in some of the He- 

 brides, have generally been referred to formations of a more re- 

 cent period. 



* History of the Mineral Kingdom, vol. ii. p. 302. 



