100 PALEOZOIC TIME. [CHAP. VI, 



likely to accumulate in waters which opened into the sea, 

 but might do so in lakes and swampy tracts which were 

 not traversed by any strong currents, and were never in- 

 vaded by the sea- waters. 



Professor Green, indeed, thinks that these beds were 

 formed in lake's which had no outlet at all, and the waters 

 of which were gradually concentrated by evaporation. He 

 points to the limestones as having characters which re- 

 semble those of limestones formed by precipitation from 

 saturated solutions, and such saturation is certainly almost 

 an impossibility in a lake with an outlet. 



There is another fact which tells greatly in favour of 

 this theory, namely, that the Upper Coal-measures are 

 frequently found to lie unconformably on those below, and 

 the Middle measures have sometimes suffered a considerable 

 amount of erosion before the upper group was deposited 

 on them. Such a relation implies terrestrial movement, 

 and probably an elevation of those districts in which it is 

 found, and when we consider the enormous extent of 

 nearly level ground which must have existed in the time of 

 the Middle Coal-measures, it is easy to see that very slight 

 uplifts would be sufficient to convert large areas into 

 shallow lake-basins. 



Moreover, the known geographical distribution of these 

 red Coal-measures has considerable significance. They 

 occur in all the Midland coalfields, as well as in Lanca- 

 shire, Yorkshire, Cumberland, and Scotland ; but the so- 

 called Upper Coal-measures of South Wales and Gloucester- 

 shire are not red, neither is there any such group in 

 Ireland or in Devonshire. It may, of course, be said that 

 the true Upper Coal-measures are absent by denudation 

 in these last-mentioned districts, but in South Wales the 

 Coal-measure series is thicker than in any other district, if 

 the upper group is deducted ; and the fact remains that 

 the red measures are only known to occur in the country 



