Parr IL. Secor. iv. §1.] CARBONIFEROUS. 719 
-insensible indeed is the gradation in many consecutive sections 
where the two systems join each other that no sharp line can there be 
drawn between them. This stratigraphical passage is likewise in 
_ many places associated with a corresponding commingling of organic 
remains, either by the ascent of undoubted Devonian species into the 
lower parts of the Carboniferous series, or by the appearance in the 
upper Devonian beds of species which attained their maximum 
development in Carboniferous times. Hence there can be no doubt 
as to the true place of the Carboniferous system in the geological 
record. In some places, however, this system is found resting 
unconformably upon Devonian or older rocks, so that local 
disturbances of considerable magnitude occurred before or at the 
commencement of the Carboniferous period. It is deserving of 
notice that Carboniferous rocks are very generally arranged in 
basin-shaped areas. This disposition, so well seen in Europe, and 
_ particularly in the central and western half of the Continent, has in 
some cases been caused merely by the plication and subsequent 
extensive denudation of what were originally wide continuous sheets 
of rock, as may be observed in the British Isles. But the remarkable 
small scattered coal-basins of France and Central Germany were 
undoubtedly from the first isolated areas of deposit, though they 
have suffered, in some cases very greatly, from subsequent plication 
and denudation. In Russia and still more in China and Western 
North America, Carboniferous rocks cover thousands of square miles 
in horizontal or only very gently undulating sheets. 
Rocks.—The materials of which the Carboniferous system is 
built up differ considerably in different regions; but two types of 
sedimentation have a wide development. In one of these, the marine 
type, limestones form the prevailing rocks, and are often visibly 
made up of organic remains, chiefly encrinites, corals, foraminifera, 
and molluscs. Sometimes these strata assume a compact homo- 
geneous character, with black, grey, white, or mottled colours, 
when they are occasionally largely quarried as marble. Local 
developments of oolitic structure occur among them. They also 
assume in some places a yellowish dull finely granular aspect and 
more or less dolomitic composition. They occur in beds sometimes, 
as in Central England and Ireland, piled over each other for a depth 
of hundreds of feet, and in Utah for several thousand feet, with little 
or no intercalation of other material than limestone. The limestones 
frequently contain irregular nodules of a white, grey, or black flinty 
chert (phtanite), which, presenting a close resemblance to the flints 
of the chalk, occur in certain beds or layers of rock, sometimes in 
numbers sufficient to form of themselves tolerably distinct strata. 
These concretions are associated with the organisms of the rock, some 
of which, completely silicified and beautifully preserved, may be 
found imbedded in the chert. Dolomite, usually of a dull yellowish 
colour, granular texture, and rough feel, occurs both in beds regularly 
interstratified with the limestones and also in broad wall-like masses 
