258 Dr Hibbert on the Limestone of Burdiehouse, 



That the existence of large fresh-water lakes is necessary to 

 explain the phenomena presented by the coal-fields of Scotland, 

 I have long since advocated. (See the Reports of the Proceedings 

 of the Royal Society of Edinburgh). The very great differences 

 of level which subsist between the coal-measures of the Great 

 Lowland Valley of Scotland, and the hills by which they are 

 bounded, namely, the higher lands north of the Tay, the great 

 Grampian chain, and the southerly grauwacke ridge which 

 stretches from St Abb's Head to Galloway, are adverse to the no- 

 tion that, during the carboniferous epoch, the continuity of 

 spacious seas was only broken by occasional groups of studded 

 islets. 



If, then, it be conceded, that such elevated lands had very 

 early risen above the level of the sea, they would rather point to 

 the existence of large continents than of archipelagos of islets ; 

 and, in admitting the development of these lofty ridges, we must 

 also infer that, acted upon by the rains of a humid atmosphere, 

 they would naturally give origin to corroding water-courses, or 

 rivers, and to large fresh- water lakes. 



Accordingly, the existence of one or more of such fresh- water 

 lakes is advocated as a very early state of the great valley of the 

 Scottish Lowlands. But it is also admitted, that this flooded 

 condition of the country might have been varied by islets ; that 

 even large tracts of dry land might have subsisted, and have been 

 invaded by arms of the sea, or estuaries ; and that while higher 

 lands would encourage the growth of such conifera? as the Craig- 

 leith fossils indicate, lesser plants, as ferns, &c. would find a soil 

 amidst marshes or shallow lakes, or on the borders of deeper fresh- 

 water basins. 



During such a condition of the globe, the calcareous deposit 

 of Burdiehouse was formed ; new races of fish inhabiting fresh- 

 waters were created, and among them the Megalichthys. The 

 ocean also must have possessed its own peculiar races, and its 



