AUSTEN — EXTENSION OF THE COAL-MEASURES. 55 



By placing the so-called, independent formations or systems — 

 the "Upper Silurian" and "Devonian" — as equivalents one of 

 another, the "Old Red Sandstone" of the British area becomes 

 disentangled from relationships which are wholly without the sup- 

 port of fossil evidence, and inconsistent with the progress and nature 

 of the physical changes of the Mid-Palseozoic period over Western 

 Europe. 



§ 2. Mountain or Carboniferous Limestone. 



No definite geological group, limited by mineral uniformity, can, 

 by itself, be the representative of a period of time ; this considera- 

 tion has, however, been too often lost sight of; and in this way the 

 "Old Red Sandstone" has been made to precede the Mountain 

 Limestone, and each has been taken to indicate distinct periods. 



The West-European area of typical Mountain Limestone — that 

 over which it presents the largest dimensions, where it is less sub- 

 divided, and purest, — admits of most precise geographical limitation. 

 It is included within an elliptical space, which may be represented 

 by a line drawn from South Wales, through Somerset and Belgium, 

 into Westphalia, and thence, through Hanover, back into Derbyshire. 

 This was dependent on one hydrographic basin. Ireland presents 

 another, and the Russian area a third. All these are synchronous 

 with one another, though very unlike in the sequence and compo- 

 sition of their subordinate beds. It requires but a very trifling 

 acquaintance with the law of change in the composition of sea-beds to 

 lead to the conclusion that the Mountain Limestone presents the ex- 

 treme outward depositions of waters charged with calcareous matter ; 

 whilst outside and beyond the area thus defined, it will be found to 

 put on characters the significance of all which is equally clear. One 

 definite form ever implies every other modification of sea-bed : in 

 this case the Mountain Limestone serves to mark the central por- 

 tion of an enclosed sea, around which must have been grouped 

 every other gradation of detrital matter up to marginal sand and 

 gravel. 



Prof. Phillips was the first to call attention to the change which 

 the Mountain Limestone series undergoes from Derbyshire into York- 

 shire, and how also, by taking three distinct sections ranging some- 

 what S. to N. within the latter county, it could be plainly seen, 

 that the group was constantly parting with its individuality from 

 below upwards, by the substitution of earthy detrital beds for cal- 

 careous ones. This process is continued on into Northumberland, 

 where the whole of the Great Scar Limestones become represented 

 by an endless alternation of shales, limestones, sandstones, and coal. 

 Whoever vsdll carefully follow out the process of change presented 

 by the Carboniferous series from our midland counties into Scotland 

 will readily satisfy himself that it must be viewed as a synchronous 

 whole, — that there is no descending order, — and that such a suppo- 

 sition, if fully set forth, implies a set of physical impossibilities, on 

 which, however, we have not space here to enlarge. The Carboni- 



