DAVIDSON — SCOTTISH CARBONIFEROUS DRACHIOPODA. 
4G5 
land, and Ireland, (geologists arc of opinion that it is susccpiible of 
being advantageously arranged into three well-marked groups, viz., 
the " Lower Coal-measures," the " Mountain- or Carboniferous-lime- 
stone," and tlu! " Upper or True Coal-measures." 
In J'higlaiid 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 Philli])S, 
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 " Cai'boniferous-limestone ;" 
and 3rdly, the " Coal-measures." But it is cliiefly 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 gi'oups, but which are not to be found 
together in eveiy district : 
, TT o 1 ^ Coal-measures. 
1. Upper Coal-measures |^ Millstone-grit. 
c -Kjr i • /-( 1 T J. f c Yoredale Rocks. 
2. Mountain or Carbonuerous-lunestone •{ 7 o t • j. 
( a bear L/imestone. 
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 intinisive 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 Cantii'e. 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 vnth coal-beds and bituminous schists in the lower parts of the 
system. 
