DAVIDSON — SCOTTISH CARBONIFEROUS BRACHIOPODA. 



465 



laud, and Ireland, geologists are of opinion that it is susceptible of 

 being advantageously arranged into three well-marked groups, viz., 

 the "Lower Coal-measures," the "Mountain- or Carboniferous-lime- 

 stone," and the " Upper or True Coal-measures." 



In England a fourth division is sometimes introduced, viz., the 

 " Millstone- grit," which is situated between the Mountain-limestone 

 and the Coal-measures, but which, according to Professor Phillips, 

 would form a kind of transition group, which may sometimes for 

 convenience be joined to the lower, sometimes to the upper, and 

 occasionally be treated alone. 



In Ireland the system has been differently divided ; but Mr. Kelly 

 is of opinion that it may be arranged into — 1st, " Lower Coal- 

 measures " (comprising the Kildress red and yellow sandstones, and 

 still higher calciferous slates) ; 2ndly, the " Carboniferous-Hmestone ;" 

 and 3rdly, the " Coal-measures." But it is chiefly the first, or lower, 

 division that predominates, and which has induced Professor Phillips 

 to assimilate the Irish carboniferous series to the great English and 

 Welsh groups. 



In England (according to the same distinguished authority) the 

 Carboniferous system, when in its most complete development, 

 would admit of the following groups, but which are not to be found 

 together in every district : 



T TT n 1 (a Coal-measures. 



1. Upper Coal-measures | ^ Millstone-grit. 



2. Mountain or Carboniferous-limestone \ ^ ^o^®^^?® Rocks. 



( a bear Limestone. 



3. Lower Coal-measures e Shales. 



Having thus briefly alluded to the divisions in England and in 

 Ireland, we may at once mention that in Scotland the three groups 

 are likewise represented. 



It has been calculated by Professor Nicol that the carboniferous 

 strata cover nearly a seventeenth of the surface of Scotland ; but it 

 is very difficult to form a correct estimate, on account of the numer- 

 ous breaks from intrusive igneous rocks rendering mapping very 

 complex. It is, however, in the central portion of Scotland that the 

 rocks which we are now describing occupy the greatest surface ; 

 they form there a wide sub-parallel band of nearly one hundred miles 

 in length, by some fifty in breadth, extending from the northern por- 

 tion of the Frith of Forth to the Clyde, and as far as the extremity 

 of Cantire. 'No portion of the system appears to have been dis- 

 covered in the north ; but in the south there exists a narrow band, 

 or separate patches, which extend along the frontiers of Scotland and 

 England, from Berwick to near Kircudbright, on the Solway Frith. 



Scottish carboniferous deposits differ, however, from strata of a 

 similar age, existing both in England and Ireland, in the manner in 

 which the various beds of encrinal- and coralline-limestones are inter- 

 calated with coal-beds and bituminous schists in the lower parts of the 

 system. 



