
-Panr IL. Sucr. iv. §2.] CARBONIFEROUS. 743 
The chert (phtanite) bands which form such marked horizons among 
these limestones are counterparts of others found abundantly in the 
_ Carboniferous Limestone of England and Scotland. They have been 
- recently studied by Messrs. Hull and Hardman, who have found them full 
of siliceous replacements of calcareous foraminifers, crinoids, &c., and who 
regard them as due to a chemical alteration on the floor of the Carboni- 
ferous sea. Portions of the limestone have a dolomitic character, and 
sometimes are oolitic. Great sheets of melaphyre, felstone, and tuff, repre- 
senting volcanic eruptions of contemporaneous date, are interpolated in 
the Carboniferous Limestone of Limerick and other parts of Ireland. 
As the limestone is traced northwards it shows a similar change to that 
which takes place in the north of England, becoming more and more 
split up with sandstone, shale, and coal-seams, until, at Ballycastle, it 
resents exactly the characters of the coal-bearing part of the formation 
in Scotland. 
Millstone Grit—This name is given to a group of sandstones and 
grits, with shales and clays, which runs persistently through the centre 
of the Carboniferous system from South Wales into the middle of 
Scotland. In South Wales it has a depth of 400 to 1000 feet; in the 
Bristol coal field, of about 1200 feet. ‘Traced northwards it is found to 
be intercalated with shales, fire-clays, and thin coals, and, like the 
lower members of the Carboniferous system, to swell out to enormous 
dimensions in the Pennine region. In North Staffordshire, according to 
Mr. Hull, it attains a thickness of 4000 feet, which in Lancashire 
increases to 5500 feet. These massive accumulations of sediment were 
deposited on the north side of a barrier of more ancient Paleozoic rocks, 
which, during all the earlier part of the Carboniferous period, seems to 
have extended across central England, and which was not submerged 
until part of the Coal-measures had been laid down. North of this 
great area of deposit the Millstone Grit thins away to not more than 
400 or 500 feet. It continues a comparatively insignificant formation 
in Scotland, attaining its greatest thickness in Janarkshire and 
_ Stirlingshire, where it is known as the Moor Rock. In Ayrshire it 
does not exist, unless its place be represented by a few beds of sandstone 
at the base of the Coal-measures. 
The Millstone Grit is generally barren of fossils. When they occur 
they are either plants like those in the coal-bearing strata above and 
below, or marine organisms of Carboniferous Limestone species. In 
Northumberland, indeed, it contains a band of limestone undistinguish- 
able from some of those in the Yoredale group and Scaur limestone. 
Coal-Measures.—This division of the Carboniferous system consists of 
numerous alternations of grey, white, yellow, sometimes reddish, sand- 
stone, dark-grey and black shales, clay-ironstones, fire-clays, and coal- 
seams. In South Wales it attains a maximum depth of about 12,000 
feet ; in the Bristol coal-field it is 5090 feet. But in these districts, as 
in the rest of the Carboniferous areas of Britain, we cannot be sure that 
all the Coal-measures originally deposited now remain, for they are 
always unconformably covered by later formations. Paleontological 
considerations, to be immediately adverted to, render it probable that 
the closing part of the Carboniferous period is not now represented in 
Britain by fossiliferous strata. Whether or not it ever was so represented 
cannot be determined, owing to the denudation which occurred before 
1 Hull’s Physical Geology and Geography of Ireland, p. 30. 
