224 
THE GEOLOGIST. 
dance of fossils ; its average thickness is about 1000 feet. The 
carboniferous slate and yellow sandstone were formerly con- 
sidered as part of the old red or Devonian system, but from the 
affinity of their fossils to those of the carboniferous deposits, 
Mr. Griffith has been induced to include them in the latter sys- 
tem. 4. The carboniferous slate contains 274 species of fossils 
of which 262 are common to the upper limestone and 65 are 
found in the Devonian deposits. 5. The yellow sandstone is most 
fully developed in the northern districts of Ireland, and sections 
of it are seen on the north coast of Donegal Bay, the northern 
shore of Lower Lough Erne, and in the counties of Fermanagh, 
Mayo, and Londonderry. On the north coast of Mayo it rests 
on beds of the old red sandstone, and attains a thickness of 1460 
feet. At Lough Erne the mica slate is covered, unconformably 
with 50 feet of red conglomerate, succeeded by yellow sandstone, 
shale, &c. 150 feet thick, containing calamites and the Modiola 
Mc Adami. This series of shale and sandstone is continued up- 
wards, through a space of 2900 feet, to the base of the lower 
limestone : it is impossible at this locality to draw the line of 
separation between the yellow sandstone and carboniferous slate, 
but the latter probably includes about 1200 feet of the whole 
mass. At several other localities the yellow sandstone also rests 
unconformably upon red conglomerate, apparently belonging to 
a distinct epoch. The most remarkable and extensive district of 
yellow sandstone in Ireland occupies the whole of the northern 
and eastern shores of Lough Foyle, in the county of London- 
derry ; its lower beds are a conglomerate, formed from the mica 
slate upon which it rests ; they are 600 feet thick, and not fos- 
silliferous. The second division of the rocks in this district, 
which is 500 feet in thickness, contains numerous beds of dark 
shale and grayish sandstone ; and the uppermost consists of red, 
greenish, and yellow shales, with numerous thin bands of 
nodular limestone and red sandstone, passing into quartoze con- 
glomerate, resembling the ordinary old red sandstone. These 
