258 Dr Hreserr 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 conifere 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 
